Illumos openzfs#1661: Fix flaw in sa_find_sizes() calculation

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.

Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Dan McDonald <[email protected]>
Approved by: Gordon Ross <[email protected]>

References to Illumos issue:
  https://www.illumos.org/issues/1661

Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#426

module/zfs/sa.c

@@ -607,14 +607,14 @@ sa_find_sizes(sa_os_t *sa, sa_bulk_attr_t *attr_desc, int attr_count,
 		 * and spill buffer.
 		 */
 		if (buftype == SA_BONUS && *index == -1 &&
-		    P2ROUNDUP(*total + hdrsize, 8) >
+		    (*total + P2ROUNDUP(hdrsize, 8)) >
 		    (full_space - sizeof (blkptr_t))) {
 			*index = i;
 			done = B_TRUE;
 		}
 
 next:
-		if (P2ROUNDUP(*total + hdrsize, 8) > full_space &&
+		if ((*total + P2ROUNDUP(hdrsize, 8)) > full_space &&
 		    buftype == SA_BONUS)
 			*will_spill = B_TRUE;
 	}