Deprecate 3des support in IPsec for FreeBSD 13.

RFC 8221 does not outright ban 3des as the algorithms deprecated for
13 in r348205, but it is listed as a SHOULD NOT and will likely be a
MUST NOT by the time 13 ships.

Discussed with:	bjk
MFC after:	1 week
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D24341

sys/netipsec/xform_esp.c

@@ -94,7 +94,7 @@ SYSCTL_VNET_PCPUSTAT(_net_inet_esp, IPSECCTL_STATS, stats,
     struct espstat, espstat,
     "ESP statistics (struct espstat, netipsec/esp_var.h");
 
-static struct timeval deswarn, blfwarn, castwarn, camelliawarn;
+static struct timeval deswarn, blfwarn, castwarn, camelliawarn, tdeswarn;
 
 static int esp_input_cb(struct cryptop *op);
 static int esp_output_cb(struct cryptop *crp);
@@ -164,6 +164,10 @@ esp_init(struct secasvar *sav, struct xformsw *xsp)
 		if (ratecheck(&deswarn, &ipsec_warn_interval))
 			gone_in(13, "DES cipher for IPsec");
 		break;
+	case SADB_EALG_3DESCBC:
+		if (ratecheck(&tdeswarn, &ipsec_warn_interval))
+			gone_in(13, "DES cipher for IPsec");
+		break;
 	case SADB_X_EALG_BLOWFISHCBC:
 		if (ratecheck(&blfwarn, &ipsec_warn_interval))
 			gone_in(13, "Blowfish cipher for IPsec");