Explain why the casting we do to silence this warning is indeed safe.

It is because the field we're casting from, though being 64-bit wide, was filled
with a pointer in first place by ourselves.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>