xfs: rename metadata inode predicates

The predicate xfs_internal_inum tells us if an inumber refers to one of
the inodes rooted in the superblock.  Soon we're going to have internal
inodes in a metadata directory tree, so this helper should be renamed
to capture its limited scope.

Ondisk inodes will soon have a flag to indicate that they're metadata
inodes.  Head off some confusion by renaming the xfs_is_metadata_inode
predicate to xfs_is_internal_inode.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>