btrfs: add a test for multi-subvolume fsyncing

I discovered a problem in btrfs where we'd end up pointing at a block we
hadn't written out yet.  This is triggered by a race when two different
files on two different subvolumes fsync.  This test exercises this path
with dm-log-writes, and then replays the log at every FUA to verify the
file system is still mountable and the log is replayable.

This test is to verify the fix

btrfs: fix incorrect updating of log root tree

actually fixed the problem.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/btrfs/group

@@ -198,3 +198,4 @@
 193 auto quick qgroup enospc limit
 194 auto volume
 195 auto volume
+196 auto metadata log volume