samples/bpf: fix xdp_monitor user output for tracepoint exception

The variable rec_i contains an XDP action code not an error.
Thus, using err2str() was wrong, it should have been action2str().

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>