Allow forced r/w mount of UFS/FFS filesystem with a bad check hash.

Normally a UFS/FFS filesystem with a bad check hash can only be
mounted read only. With this commit the mount(8) -f (force) option
can be used to force a read-write mount of a UFS/FFS filesystem with
a bad check hash. Conveniently the filesystem will proceed to
update its on-disk superblock with a corrected check hash.

(cherry picked from commit 9e9dcac95ad0a6c49a01619f82290fbf512fe25b)