bpf: fix rlimit in reuseport net selftest

Fix two issues in the reuseport_bpf selftests that were
reported by Linaro CI:

  [...]
  + ./reuseport_bpf
  ---- IPv4 UDP ----
  Testing EBPF mod 10...
  Reprograming, testing mod 5...
  ./reuseport_bpf: ebpf error. log:
  0: (bf) r6 = r1
  1: (20) r0 = *(u32 *)skb[0]
  2: (97) r0 %= 10
  3: (95) exit
  processed 4 insns
  : Operation not permitted
  + echo FAIL
  [...]
  ---- IPv4 TCP ----
  Testing EBPF mod 10...
  ./reuseport_bpf: failed to bind send socket: Address already in use
  + echo FAIL
  [...]

For the former adjust rlimit since this was the cause of
failure for loading the BPF prog, and for the latter add
SO_REUSEADDR.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://bugs.linaro.org/show_bug.cgi?id=3502


Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>