blk: fix wrong idr_pre_get() error check in loop.c

The idr_pre_get() function never returns a value < 0.  It returns 0 (no
memory) or 1 (OK).
Reported-by: Silva Paulo <psdasilva@yahoo.com>
[ Rewrote Silva's patch, but attributing it to Silva anyway  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/loop.c

@@ -1597,14 +1597,12 @@ static int loop_add(struct loop_device **l, int i)
 	struct gendisk *disk;
 	int err;
 
-	lo = kzalloc(sizeof(*lo), GFP_KERNEL);
-	if (!lo) {
 	err = -ENOMEM;
+	lo = kzalloc(sizeof(*lo), GFP_KERNEL);
+	if (!lo)
 		goto out;
-	}
 
-	err = idr_pre_get(&loop_index_idr, GFP_KERNEL);
-	if (err < 0)
+	if (!idr_pre_get(&loop_index_idr, GFP_KERNEL))
 		goto out_free_dev;
 
 	if (i >= 0) {