Fixing sdhci_send_command flooding kern.log on Raspberry Pi (at least 4)

This kind of log is continuously printed:

```
[ 2568.961452] ------------[ cut here ]------------
[ 2568.961496] WARNING: CPU: 3 PID: 32 at drivers/mmc/host/sdhci.c:1192 sdhci_send_command+0x944/0xdb8
[ 2568.961544] Modules linked in: rfcomm cmac algif_hash aes_arm64 aes_generic algif_skcipher af_alg bnep hci_uart btbcm bluetooth ecdh_generic ecc libaes 8021q garp stp llc vc4 binfmt_misc snd_soc_hdmi_codec brcmfmac drm_display_helper cec brcmutil drm_dma_helper cfg80211 rfkill drm_kms_helper v3d gpu_sched drm_shmem_helper snd_soc_core raspberrypi_hwmon snd_compress snd_bcm2835(C) snd_pcm_dmaengine bcm2835_isp(C) snd_pcm rpivid_hevc(C) i2c_brcmstb bcm2835_v4l2(C) bcm2835_codec(C) bcm2835_mmal_vchiq(C) v4l2_mem2mem videobuf2_vmalloc videobuf2_dma_contig videobuf2_memops videobuf2_v4l2 syscopyarea videobuf2_common snd_timer sysfillrect videodev vc_sm_cma(C) snd mc sysimgblt fb_sys_fops uio_pdrv_genirq nvmem_rmem uio i2c_dev drm fuse drm_panel_orientation_quirks backlight ip_tables x_tables ipv6
[ 2568.961854] CPU: 3 PID: 32 Comm: kworker/3:0H Tainted: G        WC         6.1.21-v8+ #1642
[ 2568.961866] Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
[ 2568.961876] Workqueue: kblockd blk_mq_run_work_fn
[ 2568.961896] pstate: a00000c5 (NzCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 2568.961907] pc : sdhci_send_command+0x944/0xdb8
[ 2568.961916] lr : sdhci_send_command+0x6d8/0xdb8
[ 2568.961924] sp : ffffffc00813b940
[ 2568.961929] x29: ffffffc00813b940 x28: ffffff80420b1af0 x27: 0000000000000001
[ 2568.961945] x26: 0000000000000000 x25: ffffff804016d000 x24: 0000000000000000
[ 2568.961960] x23: 0000000000000000 x22: ffffffd62e8ad858 x21: 0000000000000003
[ 2568.961975] x20: ffffff80420b1a80 x19: ffffff804016d540 x18: 0000000000000000
[ 2568.961995] x17: 0000000000000000 x16: ffffff80420bc040 x15: 0000007f8be881b8
[ 2568.962020] x14: 0000000000000000 x13: 0000000000000000 x12: 00000000443fa000
[ 2568.962046] x11: 0000000000000040 x10: ffffff8002487b78 x9 : ffffffd62db1ca44
[ 2568.962060] x8 : ffffff80420b1b38 x7 : ffffffc00813c000 x6 : 0000000000000000
[ 2568.962076] x5 : 000000001e43e000 x4 : 00000000c0000000 x3 : 0000000040000000
[ 2568.962089] x2 : d51963af78f74600 x1 : 0000000000000000 x0 : 00000000ffffffe4
[ 2568.962103] Call trace:
[ 2568.962108]  sdhci_send_command+0x944/0xdb8
[ 2568.962117]  sdhci_send_command_retry+0x48/0x150
[ 2568.962126]  sdhci_request+0x74/0xb8
[ 2568.962134]  __mmc_start_request+0x74/0x1d8
[ 2568.962145]  mmc_start_request+0x8c/0xb8
[ 2568.962154]  mmc_blk_mq_issue_rq+0x2d8/0x7f8
[ 2568.962163]  mmc_mq_queue_rq+0x118/0x2b0
[ 2568.962169]  blk_mq_dispatch_rq_list+0x324/0x8f8
[ 2568.962181]  blk_mq_do_dispatch_sched+0x2cc/0x360
[ 2568.962196]  __blk_mq_sched_dispatch_requests+0x120/0x178
[ 2568.962208]  blk_mq_sched_dispatch_requests+0x40/0x80
[ 2568.962214]  __blk_mq_run_hw_queue+0x84/0xc0
[ 2568.962222]  blk_mq_run_work_fn+0x28/0x38
[ 2568.962231]  process_one_work+0x208/0x480
[ 2568.962246]  worker_thread+0x50/0x428
[ 2568.962256]  kthread+0xfc/0x110
[ 2568.962270]  ret_from_fork+0x10/0x20
[ 2568.962285] ---[ end trace 0000000000000000 ]---
```

And this causes flooding /var/log/kern.log, which causes more and more sdhci_send_command.

This can break raspi's sdcard easily.

I searched the Internet but I ended up with finding https://forums.raspberrypi.com/viewtopic.php?t=355886 unanswered.

Instead, I decided to disable rsyslog completely. [^1]

```bash
sudo systemctl stop rsyslog syslog.socket
sudo systemctl disable rsyslog syslog.socket
sudo truncate -s 0 /var/log/kern.log /var/log/messages /var/log/syslog
```

[^1]: まあこの部分はまさかりを欲していることは間違いねぇなorz