Get the constness right, avoid nasty cast.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>