[XFS] Merge xfs_rmdir into xfs_remove

xfs_remove and xfs_rmdir are almost the same with a little more work
performed in xfs_rmdir due to the . and .. entries. This patch merges
xfs_rmdir into xfs_remove and performs these actions conditionally.

Also clean up the error handling which was a nightmare in both versions
before.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31335a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>