mm/swapfile.c: delete the "last_in_cluster < scan_base" loop in the body of scan_swap_map()

Via commit ebc2a1a69111 ("swap: make cluster allocation per-cpu"), we
can find that all SWP_SOLIDSTATE "seek is cheap"(SSD case) has already
gone to si->cluster_info scan_swap_map_try_ssd_cluster() route. So that
the "last_in_cluster < scan_base" loop in the body of scan_swap_map()
has already become a dead code snippet, and it should have been deleted.

This patch is to delete the redundant loop as Hugh and Shaohua
suggested.

[[e-mail]: fix comment, simplify code]
Signed-off-by: Chen Yucong <[e-mail]>
Cc: Shaohua Li <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/swapfile.c

···
 		/*
 		 * If seek is expensive, start searching for new cluster from
 		 * start of partition, to minimize the span of allocated swap.
-		 * But if seek is cheap, search from our current position, so
-		 * that swap is allocated from all over the partition: if the
-		 * Flash Translation Layer only remaps within limited zones,
-		 * we don't want to wear out the first zone too quickly.
+		 * If seek is cheap, that is the SWP_SOLIDSTATE si->cluster_info
+		 * case, just handled by scan_swap_map_try_ssd_cluster() above.
 		 */
-		if (!(si->flags & SWP_SOLIDSTATE))
-			scan_base = offset = si->lowest_bit;
+		scan_base = offset = si->lowest_bit;
 		last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
 
 		/* Locate the first empty (unaligned) cluster */
 		for (; last_in_cluster <= si->highest_bit; offset++) {
-			if (si->swap_map[offset])
-				last_in_cluster = offset + SWAPFILE_CLUSTER;
-			else if (offset == last_in_cluster) {
-				spin_lock(&si->lock);
-				offset -= SWAPFILE_CLUSTER - 1;
-				si->cluster_next = offset;
-				si->cluster_nr = SWAPFILE_CLUSTER - 1;
-				goto checks;
-			}
-			if (unlikely(--latency_ration < 0)) {
-				cond_resched();
-				latency_ration = LATENCY_LIMIT;
-			}
-		}
-
-		offset = si->lowest_bit;
-		last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
-
-		/* Locate the first empty (unaligned) cluster */
-		for (; last_in_cluster < scan_base; offset++) {
 			if (si->swap_map[offset])
 				last_in_cluster = offset + SWAPFILE_CLUSTER;
 			else if (offset == last_in_cluster) {