[SCTP]: Pick up an orphaned sctp_sockets_allocated counter.

This counter is currently write-only.

Drawing an analogy with the similar tcp counter, I think
that this one should be pointed by the sockets_allocated
members of sctp_prot and sctpv6_prot.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>