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)