xfs: straighten out all the naming around incore inode tree walks

We're not very consistent about function names for the incore inode
iteration function.  Turn them all into xfs_inode_walk* variants.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>