When calculating space needed for SA_BONUS buffers,
hdrsize is always rounded up to next 8-aligned boundary.
However, in two places the round up was done against
sum of 'total' plus hdrsize. On the other hand,
hdrsize increments by 4 each time, which means in
certain conditions, we would end up returning with
will_spill == 0 and (total + hdrsize) larger than
full_space, leading to a failed assertion because
it's invalid for dmu_set_bonus.
Sponsored by: iXsystems, Inc.
Reviewed by: mm
MFC after: 3 days
4aadb12e0b