xfs: clean up xchk_bmap_check_rmaps usage of XFS_IFORK_Q

XFS_IFORK_Q is supposed to be a predicate, not a function returning a
value.  Its usage is in xchk_bmap_check_rmaps is incorrect, but that
function only cares about whether or not the "size" of the data is zero
or not.  Convert that logic to use a proper boolean, and teach the
caller to skip the call entirely if the end result would be that we'd do
nothing anyway.  This avoids a crash later in this series.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[hch: generalized the NULL ifor check]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>