sctp: add dst_pending_confirm flag

sctp: add dst_pending_confirm flag

Add new transport flag to allow sockets to confirm neighbour.
When same struct dst_entry can be used for many different
neighbours we can not use it for pending confirmations.
The flag is propagated from transport to every packet.
It is reset when cached dst is reset.

Reported-by: YueHaibing <[email protected]>
Fixes: 5110effee8fd ("net: Do delayed neigh confirmation.")
Fixes: f2bb4bedf35d ("ipv4: Cache output routes in fib_info nexthops.")
Signed-off-by: Julian Anastasov <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Acked-by: Neil Horman <[email protected]>
Signed-off-by: David S. Miller <[email protected]>