After r328426, g_label depends on UFS (option FFS) code to read UFS
superblock, and the kernel will fail to link when UFS is not built
in. This commit makes it depend on a small portion of FFS bits and
thereby fixes build for this situation.

This is intended as an interim bandaid, and the actual superblock
reading code should probably be made independent of UFS, so we do
not need to depend on it (see kib@'s comment in the review for
details), and we will revisit this once the superblock check hashes
are all in place.

Differential Revision: https://reviews.freebsd.org/D14092