ixgbe: Add locking to prevent panic when setting sriov_numvfs to zero

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

ixgbe: Add locking to prevent panic when setting sriov_numvfs to zero

It is possible to disable VFs while the PF driver is processing requests
from the VF driver. This can result in a panic.

BUG: unable to handle kernel paging request at 000000000000106c
PGD 0 P4D 0
Oops: 0000 [#1] SMP NOPTI
CPU: 8 PID: 0 Comm: swapper/8 Kdump: loaded Tainted: G I --------- -
Hardware name: Dell Inc. PowerEdge R740/06WXJT, BIOS 2.8.2 08/27/2020
RIP: 0010:ixgbe_msg_task+0x4c8/0x1690 [ixgbe]
Code: 00 00 48 8d 04 40 48 c1 e0 05 89 7c 24 24 89 fd 48 89 44 24 10 83 ff
01 0f 84 b8 04 00 00 4c 8b 64 24 10 4d 03 a5 48 22 00 00 <41> 80 7c 24 4c
00 0f 84 8a 03 00 00 0f b7 c7 83 f8 08 0f 84 8f 0a
RSP: 0018:ffffb337869f8df8 EFLAGS: 00010002
RAX: 0000000000001020 RBX: 0000000000000000 RCX: 000000000000002b
RDX: 0000000000000002 RSI: 0000000000000008 RDI: 0000000000000006
RBP: 0000000000000006 R08: 0000000000000002 R09: 0000000000029780
R10: 00006957d8f42832 R11: 0000000000000000 R12: 0000000000001020
R13: ffff8a00e8978ac0 R14: 000000000000002b R15: ffff8a00e8979c80
FS: 0000000000000000(0000) GS:ffff8a07dfd00000(0000) knlGS:00000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000000106c CR3: 0000000063e10004 CR4: 00000000007726e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<IRQ>
? ttwu_do_wakeup+0x19/0x140
? try_to_wake_up+0x1cd/0x550
? ixgbevf_update_xcast_mode+0x71/0xc0 [ixgbevf]
ixgbe_msix_other+0x17e/0x310 [ixgbe]
__handle_irq_event_percpu+0x40/0x180
handle_irq_event_percpu+0x30/0x80
handle_irq_event+0x36/0x53
handle_edge_irq+0x82/0x190
handle_irq+0x1c/0x30
do_IRQ+0x49/0xd0
common_interrupt+0xf/0xf

This can be eventually be reproduced with the following script:

while :
do
echo 63 > /sys/class/net/<devname>/device/sriov_numvfs
sleep 1
echo 0 > /sys/class/net/<devname>/device/sriov_numvfs
sleep 1
done

Add lock when disabling SR-IOV to prevent process VF mailbox communication.