nbd: get config_lock before sock_shutdown

hulk inclusion
category: bugfix
bugzilla: 188799, https://gitee.com/openeuler/kernel/issues/I79QWO


CVE: NA

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Config->socks in sock_shutdown may trigger a UAF problem.
The reason is that sock_shutdown does not hold the config_lock,
so that nbd_ioctl can release config->socks at this time.

T0: NBD_SET_SOCK
T1: NBD_DO_IT

T0						T1

nbd_ioctl
  mutex_lock(&nbd->config_lock)
  // get lock
  __nbd_ioctl
	nbd_start_device_ioctl
	  nbd_start_device
	  mutex_unlock(&nbd->config_lock)
	  // relase lock
	  wait_event_interruptible
	  (kill, enter sock_shutdown)
	  sock_shutdown
					nbd_ioctl
					  mutex_lock(&nbd->config_lock)
					  // get lock
					  __nbd_ioctl
					    nbd_add_socket
					      krealloc
						kfree(p)
					        //config->socks is NULL
	    nbd_sock *nsock = config->socks // error

Fix it by moving config_lock up before sock_shutdown.

Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
(cherry picked from commit b3e3e34c)