k_mem_slab: track as a kernel object

We aren't going to allow any user mode access to the
k_mem_slab APIs, but in some cases (specifically in the
case of the I2S subsystem) we need to allow user mode
to assign a memory slab to a particular driver.

This will let us verfiy (in supervisor mode) that a provided
k_mem_slab pointer is really a k_mem_slab, and know its
initialization state, and have permissions assigned to it.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>