btrfs: add test for fsync after snapshot deletion

btrfs: add test for fsync after snapshot deletion

Test that if we delete a snapshot, delete its parent directory, create
another directory with the same name as that parent and then fsync either
the new directory or a file inside the new directory, the fsync succeeds,
the fsync log is replayable and produces a correct result.

This is motivated by a bug that is fixed by the following patch for
btrfs (linux kernel):

  Btrfs: fix unreplayable log after snapshot deletion and parent
  re-creation

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>