Fsck_ufs was using an int rather than a ufs2_daddr_t to store the
alternate superblock location when given in the -b option. When int
is 32-bits, block numbers larger than 2^32 would get truncated. This
commit changes the storage fpr the alternate superblock location
to a ufs2_daddr_t.

Submitted by: Dmitry Sivachenko <trtrmitya@gmail.com>