skd: error pointer dereference in skd_cons_disk()

My initial impulse was to check for IS_ERR_OR_NULL() but when I looked
at this code a bit more closely, we should only need to check for
IS_ERR().

The blk_mq_alloc_tag_set() returns negative error codes and zero on
success so we can just do an "if (rc) goto err_out;".  It's better to
preserve the error code anyhow.  The blk_mq_init_queue() returns error
pointers on failure, it never returns NULL.  We can also remove the
"q = NULL;" at the start because that's no longer needed.

Fixes: ca33dd92

 ("skd: Convert to blk-mq")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>