powerpc/mm: Fix the call trace when resumed from hibernation

powerpc/mm: Fix the call trace when resumed from hibernation

	In SMP mode, the kernel would produce call trace when resumed
	from hibernation. The reason is when the function destroy_context
	is called to drop the resuming mm context, the mm->context.active
	is 1 which is wrong and should be zero.
	We pass the current->active_mm as previous mm context to function
	switch_mmu_context to decrease the context.active by 1.

	In UP mode, there is no effect.

Signed-off-by: Tang Yuantian <b29983@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>