generic: add dir fsync test

generic: add dir fsync test

This test is motivated by a bug found in btrfs when replaying a
directory from the fsync log. The issue was that if a directory
entry is both found in the persisted metadata and in the fsync
log, at log replay time the directory got set with a wrong i_size.
This had the consequence of not being able to rmdir empty
directories (failed with errno ENOTEMPTY).
This was fixed in btrfs with the following linux kernel patch:

     Btrfs: fix directory recovery from fsync log

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