common/btrfs: handle dmdust as mounted device in _btrfs_buffered_read_on_mirror()

common/btrfs: handle dmdust as mounted device in _btrfs_buffered_read_on_mirror()

[BUG]
After commit ab41f0bddb73 ("common/btrfs: use _scratch_cycle_mount to
ensure all page caches are dropped"), the test case btrfs/143 can fail
like below:

 btrfs/143 6s ... [failed, exit status 1]- output mismatch (see ~/xfstests/results//btrfs/143.out.bad)
    --- tests/btrfs/143.out 2020-06-10 19:29:03.818519162 +0100
    +++ ~/xfstests/results//btrfs/143.out.bad 2023-06-19 17:04:00.575033899 +0100
    @@ -1,37 +1,6 @@
     QA output created by 143
     wrote 131072/131072 bytes
     XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................

[CAUSE]
Test case btrfs/143 uses dm-dust device to emulate read errors, this
means we can not use _scratch_cycle_mount to cycle mount $SCRATCH_MNT.

As it would go mount $SCRATCH_DEV, not the dm-dust device to
$SCRATCH_MNT.
This prevents us to trigger read-repair (since no error would be hit)
thus fail the test.

[FIX]
Since we can mount whatever device at $SCRATCH_MNT, we can not use
_scratch_cycle_mount in this case.

Instead implement a small helper to grab the mounted device and its
mount options, and use the same device and mount options to cycle
$SCRATCH_MNT mount.

This would fix btrfs/143 and hopefully future test cases which use dm
devices.

Reported-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>