xfs: fix directory inode iolock lockdep false positive

The change to add the IO lock to protect the directory extent map
during readdir operations has cause lockdep to have a heart attack
as it now sees a different locking order on inodes w.r.t. the
mmap_sem because readdir has a different ordering to write().

Add a new lockdep class for directory inodes to avoid this false
positive.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>