igc: avoid kernel warning when changing RX ring parameters

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

igc: avoid kernel warning when changing RX ring parameters

Calling ethtool changing the RX ring parameters like this:

$ ethtool -G eth0 rx 1024

on igc triggers kernel warnings like this:

[ 225.198467] ------------[ cut here ]------------
[ 225.198473] Missing unregister, handled but fix driver
[ 225.198485] WARNING: CPU: 7 PID: 959 at net/core/xdp.c:168
xdp_rxq_info_reg+0x79/0xd0
[...]
[ 225.198601] Call Trace:
[ 225.198604] <TASK>
[ 225.198609] igc_setup_rx_resources+0x3f/0xe0 [igc]
[ 225.198617] igc_ethtool_set_ringparam+0x30e/0x450 [igc]
[ 225.198626] ethnl_set_rings+0x18a/0x250
[ 225.198631] genl_family_rcv_msg_doit+0xca/0x110
[ 225.198637] genl_rcv_msg+0xce/0x1c0
[ 225.198640] ? rings_prepare_data+0x60/0x60
[ 225.198644] ? genl_get_cmd+0xd0/0xd0
[ 225.198647] netlink_rcv_skb+0x4e/0xf0
[ 225.198652] genl_rcv+0x24/0x40
[ 225.198655] netlink_unicast+0x20e/0x330
[ 225.198659] netlink_sendmsg+0x23f/0x480
[ 225.198663] sock_sendmsg+0x5b/0x60
[ 225.198667] __sys_sendto+0xf0/0x160
[ 225.198671] ? handle_mm_fault+0xb2/0x280
[ 225.198676] ? do_user_addr_fault+0x1eb/0x690
[ 225.198680] __x64_sys_sendto+0x20/0x30
[ 225.198683] do_syscall_64+0x38/0x90
[ 225.198687] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 225.198693] RIP: 0033:0x7f7ae38ac3aa

igc_ethtool_set_ringparam() copies the igc_ring structure but neglects to
reset the xdp_rxq_info member before calling igc_setup_rx_resources().
This in turn calls xdp_rxq_info_reg() with an already registered xdp_rxq_info.

Make sure to unregister the xdp_rxq_info structure first in
igc_setup_rx_resources.