qa: allow small allocation diffs for exported rbds

The local filesytem may behave slightly differently. This isn't
foolproof, but seems to be reliable enough on rhel7 rootfs, where
exact comparison was failing.

Fixes: #10002
Signed-off-by: Josh Durgin <jdurgin@redhat.com>
(cherry picked from commit e94d3c11edb9c9cbcf108463fdff8404df79be33)