sctp: remove the unnecessary transport looking up from sctp_sendmsg

Now sctp_assoc_lookup_paddr can only be called only if daddr has
been set. But if daddr has been set, sctp_endpoint_lookup_assoc
would be done, where it could already have the transport.

So this unnecessary transport looking up should be removed, but
only reset transport as NULL when SCTP_ADDR_OVER is not set for
UDP type socket.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>