tests/kernel/mem_slab: Fix memory overcommit for real

Commit 4ef36a4b ("tests/kernel/mem_slab: Fix memory overcommit")
caught this error, but missed the fact that there are two slabs that
need to be resized.  I also failed to properly explain (or, to be
honestly, fully understand) the deadlock condition, so add a nice big
comment explaining it.

Basically: you have a bunch of threads that can allocate all but one
of their blocks before trying to allocate their last one and pending.
There must be at least one block left so all the threads don't
symmetrically go to sleep waiting on each other.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>