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

[markj]

Summary

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)

[markj]

Remove unrelated diff.

[mckusick]

No change in actual running, but definitely correct change.

[kib]

In theory, can we have fs_ncg not multiple of 8 ? Even if newfs(8) can only create correctly aligned fs_ncg (but I do not see that it cares), other ways to create filesystem might make it unaligned.

So may be there is a real bug, and fixing it would make rounding fs_ncg up to sizeof(int) without NBBY division.

[kib]

In D27731#619991, @kib wrote:

In theory, can we have fs_ncg not multiple of 8 ? Even if newfs(8) can only create correctly aligned fs_ncg (but I do not see that it cares), other ways to create filesystem might make it unaligned.

So may be there is a real bug, and fixing it would make rounding fs_ncg up to sizeof(int) without NBBY division.

My concern is unfounded, howmany() rounds up.

[markj]

commit ace3d9475ceecd9bcb766bb82a1c8f87e8f560be