mm/allocpercpu.c: make 4 functions static

This patch makes the following needlessly global functions static:
 - percpu_depopulate()
 - __percpu_depopulate_mask()
 - percpu_populate()
 - __percpu_populate_mask()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>