sctp: Fix the SNMP number of SCTP_MIB_CURRESTAB

RFC3873 defined SCTP_MIB_CURRESTAB:
  sctpCurrEstab OBJECT-TYPE
    SYNTAX         Gauge32
    MAX-ACCESS     read-only
    STATUS         current
    DESCRIPTION
         "The number of associations for which the current state is
         either ESTABLISHED, SHUTDOWN-RECEIVED or SHUTDOWN-PENDING."
    REFERENCE
         "Section 4 in RFC2960 covers the SCTP   Association state
         diagram."

If the T4 RTO timer expires many times(timeout), the association will enter
CLOSED state, so we should dec the number of SCTP_MIB_CURRESTAB, not inc the
number of SCTP_MIB_CURRESTAB.

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