1. ffs_alloc() actually allowed writing one block less one frag (normally
7 frags or 7/8 blocks) beyond the limit.
2. freebufspace() gives the free space in frags, but `size' is in bytes,
so the change results in approximately `size' fragments too many being
reserved.
3. ffs_realloccg() has the same bug but wasn't changed.
PR: 3398
Submitted by: bde
Eyeballed by: phk
f6da000c05