selftests: net: Clean up an unused variable

Address compiler warning:

ip_defrag.c: In function 'send_udp_frags':
ip_defrag.c:206:16: warning: unused variable 'udphdr' [-Wunused-variable]
  struct udphdr udphdr;
                ^~~~~~
Signed-off-by: NJakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/ip_defrag.c

@@ -203,7 +203,6 @@ static void send_udp_frags(int fd_raw, struct sockaddr *addr,
 {
 	struct ip *iphdr = (struct ip *)ip_frame;
 	struct ip6_hdr *ip6hdr = (struct ip6_hdr *)ip_frame;
-	struct udphdr udphdr;
 	int res;
 	int offset;
 	int frag_len;