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

Offering: HULK
hulk inclusion
category: bugfix
bugzilla: 188902, https://gitee.com/openeuler/kernel/issues/I7EENU



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nbd->config = config and refcount_set(&nbd->config_refs, 1) in
nbd_genl_connect may be out of order, causing config_refs to be set to 1
first, and then nbd_open accessing nbd->config reports a null pointer
reference.
   T1                      T2
vfs_open
  do_dentry_open
    blkdev_open
      blkdev_get
        __blkdev_get
          nbd_open
           nbd_get_config_unlocked
                        genl_rcv_msg
                          genl_family_rcv_msg
                            genl_family_rcv_msg_doit
                              nbd_genl_connect
                                nbd_alloc_and_init_config
                                  // out of order execution
                                  refcount_set(&nbd->config_refs, 1); // 2
             nbd->config
             // null point
                                  nbd->config = config; // 1

Fix it by adding a cpu memory barrier to guarantee sequential execution.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>