sctp: prevent too-fast association id reuse

We use the idr subsystem and always ask for an id
at or above 1.  This results in a id reuse when one
association is terminated while another is created.

To prevent re-use, we keep track of the last id returned
and ask for that id + 1 as a base for each query.  We let
the idr spin lock protect this base id as well.

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