nbd: fix null-ptr-dereference while accessing 'nbd->config'

Memory reordering may occur in nbd_genl_connect(), causing config_refs
to be set to 1 while nbd->config is still empty. Opening nbd at this
time will cause null-ptr-dereference.

   T1                      T2
   nbd_open
    nbd_get_config_unlocked
                 	   nbd_genl_connect
                 	    nbd_alloc_and_init_config
                 	     //memory reordered
                  	     refcount_set(&nbd->config_refs, 1)  // 2
     nbd->config
      ->null point
			     nbd->config = config  // 1

Fix it by adding smp barrier to guarantee the execution sequence.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20231116162316.1740402-4-linan666@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>