wgengine/magicsock: fix don't fragment setsockopt arg for IPv6 on linux

Use IPV6_MTU_DISCOVER for setting don't fragment on IPv6 sockets on Linux (was
using IP_MTU_DISCOVER, the IPv4 arg).

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/peermtu_linux.go

@@ -21,7 +21,7 @@ func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
 					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
 				}
 				if network == "udp6" {
-					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
+					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
 				}
 			})
 		}