gtp: use skb_dst_update_pmtu_no_confirm() instead of direct call

skb_dst_update_pmtu_no_confirm() is a just wrapper function of
->update_pmtu(). So, it doesn't change logic

Signed-off-by: Gyeongun Kang <kyeongun15@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>