swap: tell swapon to reinitialize swap if needed

If the page size of a swap space doesn't match the page size of the
currently running kernel, swapon will fail. Let's instruct it to
reinitialize the swap space instead.

src/core/swap.c

@@ -827,7 +827,7 @@ static void swap_enter_activating(Swap *s) {
                 }
         }
 
-        r = exec_command_set(s->control_command, "/sbin/swapon", NULL);
+        r = exec_command_set(s->control_command, "/sbin/swapon", "--fixpgsz", NULL);
         if (r < 0)
                 goto fail;