samples: bpf: Allow for -ENETDOWN in xdpsock

ndo_xsk_wakeup() can return -ENETDOWN and there's no particular reason
to bail the whole application out on that case. Let's check in kick_tx()
whether errno was set to mentioned value and basically allow application
to further process frames.

Fixes: 248c7f9c

 ("samples/bpf: convert xdpsock to use libbpf for AF_XDP access")
Reported-by: Cameron Elliott <cameron@cameronelliott.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20200205045834.56795-4-maciej.fijalkowski@intel.com