samples/bpf: Fix wrong allocation size in xdp_router_ipv4_user

prefix_key->data allocates three bytes using alloca(), but four bytes are
actually accessed in the program.
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/tencent_F9E2E81922B0C181D05B96DAE5AB0ACE6B06@qq.com

samples/bpf/xdp_router_ipv4_user.c

@@ -162,7 +162,7 @@ static void read_route(struct nlmsghdr *nh, int nll)
 				__be32 gw;
 			} *prefix_value;
 
-			prefix_key = alloca(sizeof(*prefix_key) + 3);
+			prefix_key = alloca(sizeof(*prefix_key) + 4);
 			prefix_value = alloca(sizeof(*prefix_value));
 
 			prefix_key->prefixlen = 32;