sctp: avoid to repeatedly declare external variables

Move the declaration for external variables to sctp.h file avoiding
to repeatedly declare them with extern keyword.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/sctp/sctp.h
Simple merge

diff --cc net/sctp/socket.c
Simple merge

diff --cc net/sctp/sysctl.c
Simple merge