selftests/xsk: Increase chars for interface name to 16

So that "enp240s0f0" or such name can be used against xskxceiver.
While at it, also extend character count for netns name.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220901114813.16275-4-maciej.fijalkowski@intel.com