xfs: fix inode ag walk predicate function return values

There are a number of predicate functions that help the incore inode
walking code decide if we really want to apply the iteration function to
the inode.  These are boolean decisions, so change the return types to
boolean to match.

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