ffs: Avoid out-of-bounds accesses in the fs_active bitmap

We use a bitmap to track which cylinder groups have changed between
snapshot creation and filesystem suspension. The "legs" of the bitmap
are four bytes wide (see ACTIVESET()) so we must round up the allocation
size to a multiple of four bytes.

I believe this bug is harmless since UMA/kmem_* will both pad the
allocation and zero the full allocation. Note that malloc() does inline
zeroing when the allocation size is known at compile-time.

Reported by: pho (using KASAN)
Reviewed by: kib, mckusick

(cherry picked from commit ace3d9475ceecd9bcb766bb82a1c8f87e8f560be)