add missing printk loglevel in mm/swapfile.c

in mm/swapfile.c a printk() is missing a loglevel. I believe the proper
loglevel for this situation is KERN_ERR, so that's what the patch below
sets -if you agree, please apply.
Signed-off-by: NJesper Juhl <juhl-lkml@dif.dk>
Signed-off-by: NAdrian Bunk <bunk@stusta.de>

mm/swapfile.c

@@ -1442,7 +1442,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 	else if (!memcmp("SWAPSPACE2",swap_header->magic.magic,10))
 		swap_header_version = 2;
 	else {
-		printk("Unable to find swap-space signature\n");
+		printk(KERN_ERR "Unable to find swap-space signature\n");
 		error = -EINVAL;
 		goto bad_swap;
 	}