nbd: Fix NULL pointer in flush_workqueue

In the Linux kernel, the following vulnerability has been resolved:

nbd: Fix NULL pointer in flush_workqueue

Open /dev/nbdX first, the config_refs will be 1 and
the pointers in nbd_device are still null. Disconnect
/dev/nbdX, then reference a null recv_workq. The
protection by config_refs in nbd_genl_disconnect is useless.

[ 656.366194] BUG: kernel NULL pointer dereference, address: 0000000000000020
[ 656.368943] #PF: supervisor write access in kernel mode
[ 656.369844] #PF: error_code(0x0002) - not-present page
[ 656.370717] PGD 10cc87067 P4D 10cc87067 PUD 1074b4067 PMD 0
[ 656.371693] Oops: 0002 [#1] SMP
[ 656.372242] CPU: 5 PID: 7977 Comm: nbd-client Not tainted 5.11.0-rc5-00040-g76c057c84d28 #1
[ 656.373661] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
[ 656.375904] RIP: 0010:mutex_lock+0x29/0x60
[ 656.376627] Code: 00 0f 1f 44 00 00 55 48 89 fd 48 83 05 6f d7 fe 08 01 e8 7a c3 ff ff 48 83 05 6a d7 fe 08 01 31 c0 65 48 8b 14 25 00 6d 01 00 <f0> 48 0f b1 55 d
[ 656.378934] RSP: 0018:ffffc900005eb9b0 EFLAGS: 00010246
[ 656.379350] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 656.379915] RDX: ffff888104cf2600 RSI: ffffffffaae8f452 RDI: 0000000000000020
[ 656.380473] RBP: 0000000000000020 R08: 0000000000000000 R09: ffff88813bd6b318
[ 656.381039] R10: 00000000000000c7 R11: fefefefefefefeff R12: ffff888102710b40
[ 656.381599] R13: ffffc900005eb9e0 R14: ffffffffb2930680 R15: ffff88810770ef00
[ 656.382166] FS: 00007fdf117ebb40(0000) GS:ffff88813bd40000(0000) knlGS:0000000000000000
[ 656.382806] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 656.383261] CR2: 0000000000000020 CR3: 0000000100c84000 CR4: 00000000000006e0
[ 656.383819] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 656.384370] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 656.384927] Call Trace:
[ 656.385111] flush_workqueue+0x92/0x6c0
[ 656.385395] nbd_disconnect_and_put+0x81/0xd0
[ 656.385716] nbd_genl_disconnect+0x125/0x2a0
[ 656.386034] genl_family_rcv_msg_doit.isra.0+0x102/0x1b0
[ 656.386422] genl_rcv_msg+0xfc/0x2b0
[ 656.386685] ? nbd_ioctl+0x490/0x490
[ 656.386954] ? genl_family_rcv_msg_doit.isra.0+0x1b0/0x1b0
[ 656.387354] netlink_rcv_skb+0x62/0x180
[ 656.387638] genl_rcv+0x34/0x60
[ 656.387874] netlink_unicast+0x26d/0x590
[ 656.388162] netlink_sendmsg+0x398/0x6c0
[ 656.388451] ? netlink_rcv_skb+0x180/0x180
[ 656.388750] ____sys_sendmsg+0x1da/0x320
[ 656.389038] ? ____sys_recvmsg+0x130/0x220
[ 656.389334] ___sys_sendmsg+0x8e/0xf0
[ 656.389605] ? ___sys_recvmsg+0xa2/0xf0
[ 656.389889] ? handle_mm_fault+0x1671/0x21d0
[ 656.390201] __sys_sendmsg+0x6d/0xe0
[ 656.390464] __x64_sys_sendmsg+0x23/0x30
[ 656.390751] do_syscall_64+0x45/0x70
[ 656.391017] entry_SYSCALL_64_after_hwframe+0x44/0xa9

To fix it, just add if (nbd->recv_workq) to nbd_disconnect_and_put().