overlayfs: test fsync on merged directory

Test that calling fsync against a file using the merged directory does
not result in a crash nor fails unexpectedly.

This is motivated by a change in overlayfs that resulted in a crash
(invalid memory access) when the lower or upper directory belonged to
a btrfs file system. The overlayfs change came in commit 4bacc9c9234
(overlayfs: Make f_path always point to the overlay and f_inode to the
underlay). At the moment there are two patches in the linux-fsdevel
and linux-btrfs mailing lists to fix this problem:

  * vfs: add file_dentry()
  * Btrfs: fix crash/invalid memory access on fsync when using overlayfs

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

tests/overlay/group

@@ -4,3 +4,4 @@
 # - comment line before each group is "new" description
 #
 001 auto quick copyup
+002 auto quick metadata