mm/swapfile.c: explicitly show ssd/non-ssd is handled mutually exclusive

The code shows if this is ssd, it will jump to specific tag and skip the
following code for non-ssd.

Let's use "else if" to explicitly show the mutually exclusion for
ssd/non-ssd to reduce ambiguity.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/20200328060520.31449-3-richard.weiyang@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -763,9 +763,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 			goto checks;
 		else
 			goto scan;
-	}
-
-	if (unlikely(!si->cluster_nr--)) {
+	} else if (unlikely(!si->cluster_nr--)) {
 		if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
 			si->cluster_nr = SWAPFILE_CLUSTER - 1;
 			goto checks;
@@ -873,10 +871,8 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 			goto checks;
 		else
 			goto done;
-	}
-
-	/* non-ssd case, still more slots in cluster? */
-	if (si->cluster_nr && !si->swap_map[++offset]) {
+	} else if (si->cluster_nr && !si->swap_map[++offset]) {
+		/* non-ssd case, still more slots in cluster? */
 		--si->cluster_nr;
 		goto checks;
 	}