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: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210827100023.GB9449@kili
Signed-off-by: Jens Axboe <axboe@kernel.dk>