[XFS] Split xfs_dir2_leafn_lookup_int into its two pieces of functionality

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30834a
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>