badblocks: Fix a nonsense WARN_ON() which checks whether a u64 variable < 0

In _badblocks_check(), there are lines of code like this,
1246 sectors -= len;
[snipped]
1251 WARN_ON(sectors < 0);

The WARN_ON() at line 1257 doesn't make sense because sectors is
unsigned long long type and never to be <0.

Fix it by checking directly checking whether sectors is less than len.

Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Coly Li <[e-mail]>
Reviewed-by: Yu Kuai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/badblocks.c

···
 	len = sectors;
 
 update_sectors:
+	/* This situation should never happen */
+	WARN_ON(sectors < len);
+
 	s += len;
 	sectors -= len;
 
 	if (sectors > 0)
 		goto re_check;
-
-	WARN_ON(sectors < 0);
 
 	if (unacked_badblocks > 0)
 		rv = -1;