sctp: only copy the available addr data in sctp_transport_init

'addr' passed to sctp_transport_init is not always a whole size
of union sctp_addr, like the path:

  sctp_sendmsg() ->
  sctp_sendmsg_new_asoc() ->
  sctp_assoc_add_peer() ->
  sctp_transport_new() -> sctp_transport_init()

In the next patches, we will also pass the address length of data
only to sctp_assoc_add_peer().

So sctp_transport_init() should copy the only available data from
addr to peer->ipaddr, instead of 'peer->ipaddr = *addr' which may
cause slab-out-of-bounds.

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