samples/bpf: xdp_monitor, accept short options

Updated optstring parameter for getopt_long() to accept short options.
Also updated usage() function.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>