eth: alx: take rtnl_lock on resume

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

eth: alx: take rtnl_lock on resume

Zbynek reports that alx trips an rtnl assertion on resume:

RTNL: assertion failed at net/core/dev.c (2891)

RIP: 0010:netif_set_real_num_tx_queues+0x1ac/0x1c0

Call Trace:

<TASK>

__alx_open+0x230/0x570 [alx]

alx_resume+0x54/0x80 [alx]

? pci_legacy_resume+0x80/0x80

dpm_run_callback+0x4a/0x150

device_resume+0x8b/0x190

async_resume+0x19/0x30

async_run_entry_fn+0x30/0x130

process_one_work+0x1e5/0x3b0

indeed the driver does not hold rtnl_lock during its internal close

and re-open functions during suspend/resume. Note that this is not

a huge bug as the driver implements its own locking, and does not

implement changing the number of queues, but we need to silence

the splat.