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.

Change-Id: If950cfcfc63911756ac3709c8bf6da10c8b98f1b
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Git-commit: 5e6cafc83e30f0f70c79a2b7aef237dc57e29f02
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>