Address problems in blist_alloc introduced in r349777. The swap block allocator could become corrupted

if a retry to allocate swap space, after a larger allocation attempt failed, allocated a smaller set of free blocks
that ended on a 32- or 64-block boundary.

Add tests to detect this kind of failure-to-extend-at-boundary and prevent the associated accounting screwup.

Reported by: pho
Tested by: pho
Reviewed by: alc
Approved by: markj (mentor)
Discussed with: kib
Differential Revision: https://reviews.freebsd.org/D20893

sys/kern/subr_blist.c

@@ -639,15 +639,27 @@ blst_next_leaf_alloc(blmeta_t *scan, daddr_t start, int count, int maxcount)
 			 * bitpos() returns zero here.
 			 */
 			avail = blk - start + bitpos(~scan->bm_bitmap);
-			if (avail < count) {
+			if (avail < count || avail == 0) {
 				/*
 				 * There isn't a next leaf with enough free
-				 * blocks at its beginning to complete the
-				 * spanning allocation.
+				 * blocks at its beginning to bother
+				 * allocating.
 				 */
 				return (avail);
 			}
 			maxcount = imin(avail, maxcount);
+			if (maxcount % BLIST_BMAP_RADIX == 0) {
+				/*
+				 * There was no next leaf.  Back scan up to
+				 * last leaf.
+				 */
+				--scan;
+				while (radix != BLIST_BMAP_RADIX) {
+					radix /= BLIST_META_RADIX;
+					--scan;
+				}
+				blk -= BLIST_BMAP_RADIX;
+			}
 		}
 	}