wgengine/magicsock: simplify tryEnableUDPOffload() (#9872)

Don't assume Linux lacks UDP_GRO support if it lacks UDP_SEGMENT
support. This mirrors a similar change in wireguard/wireguard-go@177caa7
for consistency sake. We haven't found any issues here, just being
overly paranoid.

Updates #cleanup

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/magicsock_linux.go

@@ -328,11 +328,7 @@ func tryEnableUDPOffload(pconn nettype.PacketConn) (hasTX bool, hasRX bool) {
 		}
 		err = rc.Control(func(fd uintptr) {
 			_, errSyscall := syscall.GetsockoptInt(int(fd), unix.IPPROTO_UDP, unix.UDP_SEGMENT)
-			if errSyscall != nil {
-				// no point in checking RX, TX support was added first.
-				return
-			}
-			hasTX = true
+			hasTX = errSyscall == nil
 			errSyscall = syscall.SetsockoptInt(int(fd), unix.IPPROTO_UDP, unix.UDP_GRO, 1)
 			hasRX = errSyscall == nil
 		})