xdp/trace: extend tracepoint in devmap with an err

Extending tracepoint xdp:xdp_devmap_xmit in devmap with an err code
allow people to easier identify the reason behind the ndo_xdp_xmit
call to a given driver is failing.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>