sh: Kill off pmb slab cache destructor.

This is the last remaining slab destructor in the kernel, which
we kill off and move the resultant list tracking logic up to
the pmb_alloc()/pmb_free() paths.

As Christoph Lameter pointed out, it's potentially unsafe to be
taking the list lock in the destructor anyways, so this is also
more fundamentally correct.

With this in place, we're all set for killing off slab destructors
from the kernel entirely.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>