xfs/185: don't fail when rtfile is larger than rblocks

This test creates a 200MB rt volume on a file-backed loopdev.  However,
if the size of the loop file is not congruent with the rt extent size
(e.g.  28k) then the rt volume will not use all 200MB because we cannot
have partial rt extents.  Because of this rounding, we can end up with
an fsmap listing that covers fewer sectors than the bmap of the loop
file.

Fix the test to allow this case.

Cc: fstests@vger.kernel.org # v2022.05.01
Fixes: 410a2e3186a1e8 ("xfs: regresion test for fsmap problems with realtime")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>