selftests/net: Fix reuseport_bpf_numa by skipping unavailable nodes

In some platforms the numa node numbers are not necessarily consecutive,
meaning that not all nodes from 0 to the value returned by numa_max_node()
are available on the system. Using node numbers which are not available
results on errors from libnuma such as:

  ---- IPv4 UDP ----
  send node 0, receive socket 0
  libnuma: Warning: Cannot read node cpumask from sysfs
  ./reuseport_bpf_numa: failed to pin to node: No such file or directory

Fix it by checking if the node number bit is set on numa_nodes_ptr, which
is defined on libnuma as "Set with all nodes the kernel has exposed to
userspace".

Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211101145317.286118-1-kleber.souza@canonical.com