swaponsomething() skips the first 2 pages.
Subtract these 2 pages from sw_nblcks as well as from
swap_pager_avail and swap_total to keep track the correct numbers.

Given sw_used and sw_nblcks now keeps track of correct pages,
we can use these to check remaining pages blist_alloc().
This improves situation where some of swap devices are filled.

Index: sys/vm/swap_pager.c
===================================================================
--- sys/vm/swap_pager.c	(revision 324645)
+++ sys/vm/swap_pager.c	(working copy)
@@ -131,6 +130,12 @@
 #define	SWAP_META_PAGES		PCTRIE_COUNT
 
 /*
+ * SWAP_DEVICE_START_OFFSET is to skip first two blocks in order to avoid
+ * overwriting any bsd label at the front of the partition.
+ */
+#define	SWAP_DEVICE_START_OFFSET	2
+
+/*
  * A swblk structure maps each page index within a
  * SWAP_META_PAGES-aligned and sized range to the address of an
  * on-disk swap block (or SWAPBLK_NONE). The collection of these
@@ -693,7 +753,7 @@
 	for (i = 0; i < nswapdev; i++) {
 		if (sp == NULL)
 			sp = TAILQ_FIRST(&swtailq);
-		if (!(sp->sw_flags & SW_CLOSING)) {
+		if (!(sp->sw_flags & SW_CLOSING) && (sp->sw_nblks - sp->sw_used >= npages)) {
 			blk = blist_alloc(sp->sw_blist, npages);
 			if (blk != SWAPBLK_NONE) {
 				blk += sp->sw_first;
@@ -2169,11 +2257,12 @@
 	}
 
 	sp = malloc(sizeof *sp, M_VMPGDATA, M_WAITOK | M_ZERO);
+	/* Can this fail?  */
 	sp->sw_vp = vp;
 	sp->sw_id = id;
 	sp->sw_dev = dev;
 	sp->sw_flags = 0;
-	sp->sw_nblks = nblks;
+	sp->sw_nblks = nblks - SWAP_DEVICE_START_OFFSET;
 	sp->sw_used = 0;
 	sp->sw_strategy = strategy;
 	sp->sw_close = close;
@@ -2180,11 +2269,12 @@
 	sp->sw_flags = flags;
 
 	sp->sw_blist = blist_create(nblks, M_WAITOK);
+	/* Can thsi fail? */
 	/*
 	 * Do not free the first two block in order to avoid overwriting
 	 * any bsd label at the front of the partition
 	 */
-	blist_free(sp->sw_blist, 2, nblks - 2);
+	blist_free(sp->sw_blist, SWAP_DEVICE_START_OFFSET, sp->sw_nblks);
 
 	dvbase = 0;
 	mtx_lock(&sw_dev_mtx);
@@ -2202,8 +2296,8 @@
 	sp->sw_end = dvbase + nblks;
 	TAILQ_INSERT_TAIL(&swtailq, sp, sw_list);
 	nswapdev++;
-	swap_pager_avail += nblks - 2;
-	swap_total += (vm_ooffset_t)nblks * PAGE_SIZE;
+	swap_pager_avail += sp->sw_nblks;
+	swap_total += (vm_ooffset_t)(sp->sw_nblks) * PAGE_SIZE;
 	swapon_check_swzone();
 	swp_sizecheck();
 	mtx_unlock(&sw_dev_mtx);
@@ -2298,7 +2393,7 @@
 	 */
 	mtx_lock(&sw_dev_mtx);
 	sp->sw_flags |= SW_CLOSING;
-	swap_pager_avail -= blist_fill(sp->sw_blist, 0, nblks);
+	swap_pager_avail -= blist_fill(sp->sw_blist, SWAP_DEVICE_START_OFFSET, nblks);
 	swap_total -= (vm_ooffset_t)nblks * PAGE_SIZE;
 	mtx_unlock(&sw_dev_mtx);