nbd: fix uaf in nbd_open

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I81XCK



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Commit bd0c825c26c3 ("[Backport] nbd: use blk_mq_alloc_disk and
blk_cleanup_disk") cleans up disk by blk_cleanup_disk() and it won't set
disk->private_data as NULL as before. UAF may be triggered in nbd_open()
if someone tries to open nbd device right after nbd_put() since refcount
of nbd device is zero and private_data is not NULL.

Fixes: bd0c825c26c3 ("[Backport] nbd: use blk_mq_alloc_disk and blk_cleanup_disk")
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Signed-off-by: Li Nan <linan122@huawei.com>