Update parsing of btrfs filesystem show for the UUID (#733601)

Issue 1/2 - GParted shows warnings for mounted btrfs file systems

'btrfs filesystem show /dev/PTN' command is used to query details of a
btrfs file system.  When the file system is mounted the command reports
failed exit status 1, but only when using btrfs-progs v3.14 and v3.14.1.
This causes GParted to: (1) report warnings from the failed commands for
a mounted btrfs file system, (2) fail to determine file system usage
figures and (3) fail to display the mount point and busy indicator for
non-mounting devices in multi-device btrfs file systems.  The label is
also read using the secondary blkid method via the FS_Info cache.

Failed exit status 1 when the btrfs file system is mounted:

    # fgrep sdb1 /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,seclabel,relatime,space_cache 0 0
    # btrfs filesystem show /dev/sdb1
    Label: 'test1-btrfs'  uuid: 033e6b07-ee6a-4620-a585-8580a2b83275
            Total devices 1 FS bytes used 192.00KiB
            devid    1 size 2.00GiB used 240.75MiB path /dev/sdb1

    Btrfs v3.14.1
    # echo $?
    1

Successful exit status 0 when the btrfs file system is unmounted:

    # umount /dev/sdb1
    # btrfs filesystem show /dev/sdb1
    Label: 'test1-btrfs'  uuid: 033e6b07-ee6a-4620-a585-8580a2b83275
            Total devices 1 FS bytes used 192.00KiB
            devid    1 size 2.00GiB used 240.75MiB path /dev/sdb1

    Btrfs v3.14.1
    # echo $?
    0

Fix by ignoring the exit status of the 'btrfs filesystem show' command
and rely on parsing the required information to determine success or
failure.  The output from the older 'btrfs-show' command is almost
identical so the same code will parse it in all cases.

Patch 1/4 - btrfs::read_uuid()

This patch changes the secondary method used to read the btrfs UUID to
resolve issue 1/2 as described above.

Bug #733601 - Btrfs: Warnings and missing label with btrfs-progs 3.12
              and 3.14
Update parsing of btrfs filesystem show for the label (#733601)

Issue 2/2 - GParted doesn't show label for mounted btrfs file systems

'btrfs filesystem show /dev/PTN' command is used to query details of a
btrfs file system including reading the file system label.  When the
file system is mounted the label is no longer enclosed in single quotes,
but only when using btrfs-progs v3.12.  This causes GParted to think the
label is blank when the file system is mounted and therefore no longer
display it.

File system label not enclosed in single quotes when mounted:

    # fgrep sdb1 /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,relatime,space_cache 0 0
    # btrfs filesystem show /dev/sdb1
    Label: test1-btrfs  uuid: 1f78fa38-2f85-41d3-9be6-ae0356ae9469
            Total devices 1 FS bytes used 192.00KiB
            devid    1 size 2.00GiB used 240.75MiB path /dev/sdb1

    Btrfs v3.12

File system label enclosed in single quotes when unmounted:

    # umount /dev/sdb1
    # btrfs filesystem show /dev/sdb1
    Label: 'test1-btrfs'  uuid: 1f78fa38-2f85-41d3-9be6-ae0356ae9469
            Total devices 1 FS bytes used 192.00KiB
            devid    1 size 2.00GiB used 240.75MiB path /dev/sdb1

    Btrfs v3.12

Removing the single quotes enclosing the label makes the output
identical to that from the older 'btrfs-show' command.

Fix by using a common parser to extract the label from both the
'btrfs filesystem show' and 'btrfs-show' commands which can read the
label with and without enclosing single quotes.

Patch 2/4 - btrfs::read_label()

This patch changes the btrfs file system label parsing code to resolve
issue 1/2 by ignoring the exit status from the 'btrfs filesystem show'
command and relying on parsing the required information to determine
success or failure.  Issue 2/2 is also resolved as described above.

Bug #733601 - Btrfs: Warnings and missing label with btrfs-progs 3.12
              and 3.14
Update parsing of btrfs filesystem show for file system usage (#733601)

Patch 3/4 - btrfs::read_label()

This patch changes the btrfs file system usage reading code to resolve
issue 1/2 by ignoring the exit status from the 'btrfs filesystem show'
command and relying on parsing the required information to determine
success or failure.

Bug #733601 - Btrfs: Warnings and missing label with btrfs-progs 3.12
              and 3.14
Update parsing of btrfs filesystem show for multi-device membership (#733601)

This patch changes the reading of the btrfs multi-device membership to
resolve issue 1/2 by ignoring the exit status from the 'btrfs filesystem
show' command and relying on parsing the required information to
determine success or failure.

Bug #733601 - Btrfs: Warnings and missing label with btrfs-progs 3.12
              and 3.14