[SCTP] net/sctp/socket.c: make 3 variables static

This patch makes the following needlessly global variables static:
- sctp_memory_pressure
- sctp_memory_allocated
- sctp_sockets_allocated

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>