ipv6: datagram: Refactor dst lookup and update codes to a new function

This patch moves the route lookup and update codes for connected
datagram sk to a newly created function ip6_datagram_dst_update()

It will be reused during the pmtu update in the later patch.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>