SCTP: Fix a potential race between timers and receive path.

There is a possible race condition where the timer code will
free the association and the next packet in the queue will also
attempt to free the same association.

The example is, when we receive an ABORT at about the same time
as the retransmission timer fires.  If the timer wins the race,
it will free the association.  Once it releases the lock, the
queue processing will recieve the ABORT and will try to free
the association again.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>