selftests/net: fix warnings on reuseaddr_ports_exhausted

Fix multiple warnings seen with gcc 10.2.1:
reuseaddr_ports_exhausted.c:32:41: warning: missing braces around initializer [-Wmissing-braces]
   32 | struct reuse_opts unreusable_opts[12] = {
      |                                         ^
   33 |  {0, 0, 0, 0},
      |   {   } {   }

Fixes: 7f204a7d ("selftests: net: Add SO_REUSEADDR test to check if 4-tuples are fully utilized.")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>