Fix IP_MTU_DISCOVER typo

The code in bss_dgram.c checks if IP_MTUDISCOVER is defined, where it
should test for IP_MTU_DISCOVER

RT#3888
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/bio/bss_dgram.c

@@ -882,7 +882,7 @@ static long dgram_ctrl(BIO *b, int cmd, long num, void *ptr)
                 perror("setsockopt");
                 ret = -1;
             }
-# elif defined(OPENSSL_SYS_LINUX) && defined(IP_MTUDISCOVER)
+# elif defined(OPENSSL_SYS_LINUX) && defined(IP_MTU_DISCOVER)
             if ((sockopt_val = num ? IP_PMTUDISC_PROBE : IP_PMTUDISC_DONT),
                 (ret = setsockopt(b->num, IPPROTO_IP, IP_MTU_DISCOVER,
                                   &sockopt_val, sizeof(sockopt_val))) < 0) {