Relax checking if the privider size matches size recorded in the
superblock, allowing provider to be bit bigger, i.e. have some
extra padding after the FS image. That in some cases might be
a side-effect of using CLOOP format which enforces certain block
size and trying to compress image that is not exactly the number
of those blocks in size. The UFS itself does not have any issues
mounting such padded file systems, so it's what GEOM_LABEL should
do.

Submitted by: @mizhka_gmail.com
Differential Revision: https://reviews.freebsd.org/D6208