selftests: xsk: Specify number of sockets to create

Add the ability in the test specification to specify numbers of
sockets to create. The default is one socket. This is then used to
remove test specific if-statements around the bpf_res tests.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-12-magnus.karlsson@gmail.com