Properly destroy work items in spl_kmem_cache_destroy()

In a non-debug build the ASSERT() would be optimized away
which could cause pending work items to not be cancelled.

We must also use cancel_delayed_work_sync() rather than just
cancel_delayed_work() to actually wait until work items have
completed. Otherwise they might accidentally access free'd
memory.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes ZFS bugs #279, #62, #363, #418