[SCTP]: Move sysctl_sctp_[rw]mem definitions to protocol.c

The sctp_[rw]mem definitions should really be in protocol.c
since that is where they are initialized.  This also allows
one to build a kernel without sysctl support.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>