ipv6: allow to cache dst for a connected sk in ip6_sk_dst_lookup_flow()

Add 'connected' parameter to ip6_sk_dst_lookup_flow() and update
the cache only if ip6_sk_dst_check() returns NULL and a socket
is connected.

The function is used as before, the new behavior for UDP sockets
in udpv6_sendmsg() will be enabled in the next patch.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>