mm: vmalloc: use const void * for caller argument

'const void *' is a safer type for caller function type. This patch
updates all references to caller function type.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>