udp gso/gro: replace hard-coded 1472 with DEFAULT_UDP_BLKSIZE fallback

Use the existing DEFAULT_UDP_BLKSIZE for GSO datagram-size fallback instead of the literal 1472.

Rationale: avoids a magic number and keeps a conservative, widely safe default across IPv4/IPv6 when the control socket MSS is unavailable. In normal operation UDP blksize is derived from the control TCP MSS; this constant fallback is only used when MSS cannot be determined or when the computed gso_dg_size ends up 0 (unlimited case).

Behavior notes:

- If the user sets -l/--length, that value drives both UDP block size and gso_dg_size.

- Otherwise, gso_dg_size tracks the chosen blksize; it falls back to DEFAULT_UDP_BLKSIZE only when the computed value is 0.

Files:

- src/iperf_api.c: update two fallback sites

- src/iperf_client_api.c: update fallback site

Author: gegles

src/iperf_api.c

@@ -1931,7 +1931,7 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
             test->settings->gso_bf_size = (test->settings->gso_bf_size / test->settings->gso_dg_size) * test->settings->gso_dg_size;
         } else {
             /* If gso_dg_size is 0 (unlimited bandwidth), use default UDP datagram size */
-            test->settings->gso_dg_size = 1472; /* Standard UDP payload size for Ethernet MTU */
+            test->settings->gso_dg_size = DEFAULT_UDP_BLKSIZE;
         }
     }
 #endif
@@ -2610,7 +2610,7 @@ get_parameters(struct iperf_test *test)
 	        test->settings->gso_bf_size = (test->settings->gso_bf_size / test->settings->gso_dg_size) * test->settings->gso_dg_size;
 	    } else {
 	        /* If gso_dg_size is 0 (unlimited bandwidth), use default UDP datagram size */
-	        test->settings->gso_dg_size = 1472; /* Standard UDP payload size for Ethernet MTU */
+	        test->settings->gso_dg_size = DEFAULT_UDP_BLKSIZE;
 	    }
 	}
 #endif

src/iperf_client_api.c

@@ -524,7 +524,7 @@ iperf_connect(struct iperf_test *test)
             test->settings->gso_bf_size = (test->settings->gso_bf_size / test->settings->gso_dg_size) * test->settings->gso_dg_size;
         } else {
             /* If gso_dg_size is 0 (unlimited bandwidth), use default UDP datagram size */
-            test->settings->gso_dg_size = 1472; /* Standard UDP payload size for Ethernet MTU */
+            test->settings->gso_dg_size = DEFAULT_UDP_BLKSIZE;
         }
     }
 #endif