powerpc/mm: Fix assert_pte_locked to work properly on uniprocessor

Since the pte_lockptr is a spinlock it gets optimized away on
uniprocessor builds so using spin_is_locked is not correct.  We can use
assert_spin_locked instead and get the proper behavior between UP and
SMP builds.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>