dmesg でデバイスファイル名等確認

## 概要
`dmesg` はカーネルが出力したメッセージを表示するコマンド。

SDカードのマウント等に関係したメッセージを確認することで、例えばマウントでエラーが起きていないかなど確認できるっぽい。

他にもデバイスファイル名の確認にもなるかも？

## 例
SDカードを挿したとき。
### 成功
```
hoge@fuga:~# dmesg | grep "mmcblk"
[    2.263781] mmcblk mmc0:aaaa: no of_node; not parsing pinctrl DT
[    2.265613] mmcblk0: mmc0:aaaa SL16G 14.8 GiB
[    2.269669]  mmcblk0: p1
```

### エラー
```
hoge@fuga:~# dmesg | grep "mmcblk"
[    2.264323] mmcblk mmc0:1234: no of_node; not parsing pinctrl DT
[    2.265635] mmcblk0: mmc0:1234 SA16G 14.4 GiB
[    2.268000]  mmcblk0: p1
[   16.043423] print_req_error: I/O error, dev mmcblk0, sector 8192
[   16.049477] Buffer I/O error on dev mmcblk0p1, logical block 0, lost sync page write
[   29.127348] print_req_error: I/O error, dev mmcblk0, sector 12692
[   29.133562] Buffer I/O error on dev mmcblk0p1, logical block 4500, lost async page write
[   29.142936] print_req_error: I/O error, dev mmcblk0, sector 16384
[   29.149151] Buffer I/O error on dev mmcblk0p1, logical block 8192, lost async page write
[   29.158802] print_req_error: I/O error, dev mmcblk0, sector 9000
[   29.164846] Buffer I/O error on dev mmcblk0p1, logical block 808, lost async page write
[   29.174049] print_req_error: I/O error, dev mmcblk0, sector 16704
[   29.180168] Buffer I/O error on dev mmcblk0p1, logical block 8512, lost async page write
[   29.188277] Buffer I/O error on dev mmcblk0p1, logical block 8513, lost async page write
[   29.196382] Buffer I/O error on dev mmcblk0p1, logical block 8514, lost async page write
[   29.204487] Buffer I/O error on dev mmcblk0p1, logical block 8515, lost async page write
[   29.212591] Buffer I/O error on dev mmcblk0p1, logical block 8516, lost async page write
[   29.220705] Buffer I/O error on dev mmcblk0p1, logical block 8517, lost async page write
[   29.228818] Buffer I/O error on dev mmcblk0p1, logical block 8518, lost async page write
[   29.238277] print_req_error: I/O error, dev mmcblk0, sector 8193
[  239.536822] print_req_error: I/O error, dev mmcblk0, sector 9000
[  239.543029] Buffer I/O error on dev mmcblk0p1, logical block 808, lost async page write
[  239.551902] print_req_error: I/O error, dev mmcblk0, sector 12692
[  239.558132] Buffer I/O error on dev mmcblk0p1, logical block 4500, lost async page write
[  239.567829] print_req_error: I/O error, dev mmcblk0, sector 16704
[  239.574106] Buffer I/O error on dev mmcblk0p1, logical block 8512, lost async page write
[  239.583232] print_req_error: I/O error, dev mmcblk0, sector 16896
[  239.589458] Buffer I/O error on dev mmcblk0p1, logical block 8704, lost async page write
[  239.597644] Buffer I/O error on dev mmcblk0p1, logical block 8705, lost async page write
[  239.605801] Buffer I/O error on dev mmcblk0p1, logical block 8706, lost async page write
[  239.613962] Buffer I/O error on dev mmcblk0p1, logical block 8707, lost async page write
[  239.622105] Buffer I/O error on dev mmcblk0p1, logical block 8708, lost async page write
[  239.630219] Buffer I/O error on dev mmcblk0p1, logical block 8709, lost async page write
[  239.638486] Buffer I/O error on dev mmcblk0p1, logical block 8710, lost async page write
[  239.647843] print_req_error: I/O error, dev mmcblk0, sector 16768
[  239.655300] print_req_error: I/O error, dev mmcblk0, sector 17024
[  239.664076] print_req_error: I/O error, dev mmcblk0, sector 8193
[  239.670179] print_req_error: I/O error, dev mmcblk0, sector 16384
[  239.677324] print_req_error: I/O error, dev mmcblk0, sector 16960
[  239.683455] print_req_error: I/O error, dev mmcblk0, sector 16896
[  249.708583] print_req_error: I/O error, dev mmcblk0, sector 17088
[  249.716507] print_req_error: I/O error, dev mmcblk0, sector 8193
[  249.722559] print_req_error: I/O error, dev mmcblk0, sector 9000
[  249.722581] Buffer I/O error on dev mmcblk0p1, logical block 808, lost async page write
[  249.736803] Buffer I/O error on dev mmcblk0p1, logical block 1, lost async page write
[  249.745672] print_req_error: I/O error, dev mmcblk0, sector 12692
[  249.751796] print_req_error: I/O error, dev mmcblk0, sector 17024
[  249.751812] Buffer I/O error on dev mmcblk0p1, logical block 8832, lost async page write
[  249.766156] Buffer I/O error on dev mmcblk0p1, logical block 4500, lost async page write
[  259.791444] print_req_error: I/O error, dev mmcblk0, sector 17109
[  259.797798] Buffer I/O error on dev mmcblk0p1, logical block 8917, lost async page write
[  259.806008] Buffer I/O error on dev mmcblk0p1, logical block 8918, lost async page write
[  259.814269] Buffer I/O error on dev mmcblk0p1, logical block 8919, lost async page write
[  259.823526] print_req_error: I/O error, dev mmcblk0, sector 17024
[  259.829771] Buffer I/O error on dev mmcblk0p1, logical block 8832, lost async page write
[  266.856983] FAT-fs (mmcblk0p1): error, fat_get_cluster: invalid cluster chain (i_pos 141315)
[  266.865528] FAT-fs (mmcblk0p1): Filesystem has been set read-only
[  266.872571] FAT-fs (mmcblk0p1): error, fat_free: invalid cluster chain (i_pos 141315)
[  266.881017] FAT-fs (mmcblk0p1): error, fat_get_cluster: invalid cluster chain (i_pos 141315)
[  266.889741] FAT-fs (mmcblk0p1): error, fat_free: invalid cluster chain (i_pos 141315)
[  268.255816] FAT-fs (mmcblk0p1): error, fat_get_cluster: invalid cluster chain (i_pos 141315)
[  268.264630] FAT-fs (mmcblk0p1): error, fat_free: invalid cluster chain (i_pos 141315)
[  269.858137] print_req_error: I/O error, dev mmcblk0, sector 17135
[  269.864602] Buffer I/O error on dev mmcblk0p1, logical block 8943, lost async page write
[  269.874095] print_req_error: I/O error, dev mmcblk0, sector 9000
[  269.880309] Buffer I/O error on dev mmcblk0p1, logical block 808, lost async page write
[  269.889609] print_req_error: I/O error, dev mmcblk0, sector 12692
[  269.896019] Buffer I/O error on dev mmcblk0p1, logical block 4500, lost async page write
[  269.905515] print_req_error: I/O error, dev mmcblk0, sector 17024
[  269.911878] Buffer I/O error on dev mmcblk0p1, logical block 8832, lost async page write
[  269.921618] print_req_error: I/O error, dev mmcblk0, sector 8193
[  269.927857] Buffer I/O error on dev mmcblk0p1, logical block 1, lost async page write
hoge@fuga:~#
```