xdp: add tracepoints for XDP mem

These tracepoints make it easier to troubleshoot XDP mem id disconnect.

The xdp:mem_disconnect tracepoint cannot be replaced via kprobe. It is
placed at the last stable place for the pointer to struct xdp_mem_allocator,
just before it's scheduled for RCU removal. It also extract info on
'safe_to_remove' and 'force'.

Detailed info about in-flight pages is not available at this layer. The next
patch will added tracepoints needed at the page_pool layer for this.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>