Fix kmem:slab_overcommit regression test locking

Fix kmem:slab_overcommit regression test locking

This regression test could crash in splat_kmem_cache_test_reclaim()
due to a race between the slab relclaim and the normal exiting of
the thread.  Specifically, the kct structure could be free'd by
the thread performing the allocations while the reclaim function
was also working on that's threads kct structure.  The simplest
fix is to extend the kcp->kcp_lock over the reclaim to prevent
the kct from being freed.  A better fix would be to ref count
these structures, but since is just a regression this locking
change is enough.  Surprisingly this was only observed commonly
under RHEL5.4 but all platform could have hit this.