[SCTP]: Beginning of sin_port fixes.

That's going to be a long series.  Introduced temporary helpers
doing copy-and-convert for sctp_addr; they are used to kill
flip-in-place in global data structures and will be used
to gradually push host-endian uses of sctp_addr out of existence.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>