pd: fix a NULL vs IS_ERR() check

blk_mq_alloc_disk() returns error pointers, it doesn't return NULL
so correct the check.

Fixes: 262d431f ("pd: use blk_mq_alloc_disk and blk_cleanup_disk")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210827100023.GB9449@kiliSigned-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/paride/pd.c

@@ -892,7 +892,7 @@ static void pd_probe_drive(struct pd_unit *disk)
 		return;
 
 	p = blk_mq_alloc_disk(&disk->tag_set, disk);
-	if (!p) {
+	if (IS_ERR(p)) {
 		blk_mq_free_tag_set(&disk->tag_set);
 		return;
 	}