gve: Fix Tx tcpdump panic

[shailend_google.com]

Summary

Ringing the doorbell before making the BPF call can result in the
mbuf being freed before the BPF call.

[markj]

sys/dev/gve/gve_tx.c

709

BTW, this usage of drbr looks like it could be a bit simpler:

while ((if_getdrvflags(ifp) & IFF_DRV_RUNNING) != 0 && (m = drbr_peek()) != NULL) {
    if (gve_xmit(tx, mbuf) != 0) {
        ...
    }
    drbr_advance(ifp, tx->br);
    ...
}

Right now we're needlessly checking for an empty ring in drbr_peek() after having already done so with drbr_empty().