sfc: last resort fallback for lack of xdp tx queues

Previous patch addressed the situation of having some free resources for
xdp tx but not enough for one tx queue per CPU. This patch address the
worst case of not having resources at all for xdp tx.

Instead of using queues dedicated to xdp, normal queues used by network
stack are shared for both cases, using __netif_tx_lock for
synchronization. Also queue stop/restart must be considered in the xdp
path to avoid freezing the queue.

This is not the ideal situation we might want to be, and a performance
penalty is expected both for normal and xdp traffic, but at least XDP
will work in all possible situations (with a warning in the logs),
improving a bit the pain of not knowing in what situations we can use it
and in what situations we cannot.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>