net: socket: suppress unused warning

suppress unused warnings and fix the error that there is
with the W=1 enabled.

Warning generated

net/socket.c: In function ‘__sys_getsockopt’:
net/socket.c:2300:13: error: variable ‘max_optlen’ set but not used [-Werror=unused-but-set-variable]
 2300 |         int max_optlen;
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230310221851.304657-1-vincenzopalazzodev@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/socket.c

@@ -2292,9 +2292,9 @@ INDIRECT_CALLABLE_DECLARE(bool tcp_bpf_bypass_getsockopt(int level,
 int __sys_getsockopt(int fd, int level, int optname, char __user *optval,
 		int __user *optlen)
 {
+	int max_optlen __maybe_unused;
 	int err, fput_needed;
 	struct socket *sock;
-	int max_optlen;
 
 	sock = sockfd_lookup_light(fd, &err, &fput_needed);
 	if (!sock)