xfs: split the xchk_bmap_check_rmaps into a predicate

This function has two parts: the second part scans every reverse mapping
record for this file fork to make sure that there's a corresponding
mapping in the fork, and the first part decides if we even want to do
that.

Split the first part into a separate predicate so that we can make more
changes to it in the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>