sfc: trace_xdp_exception on XDP failure

The sfc driver can drop packets processed with XDP, notably when running
out of buffer space on XDP_TX, or returning an unknown XDP action.
This increments the rx_xdp_bad_drops ethtool counter.

Call trace_xdp_exception everywhere rx_xdp_bad_drops is incremented,
except for fragmented RX packets as the XDP program hasn't run yet.
This allows it to easily be monitored from userspace.

This mirrors the behavior of other drivers.

Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>