powerpc/mm: Fix a AB->BA deadlock scenario with nohash MMU context lock

The MMU context_lock can be taken from switch_mm() while the
rq->lock is held. The rq->lock can also be taken from interrupts,
thus if we get interrupted in destroy_context() with the context
lock held and that interrupt tries to take the rq->lock, there's
a possible deadlock scenario with another CPU having the rq->lock
and calling switch_mm() which takes our context lock.

The fix is to always ensure interrupts are off when taking our
context lock. The switch_mm() path is already good so this fixes
the destroy_context() path.

While at it, turn the context lock into a new style spinlock.

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