[PATCH] reiserfs: fix 32-bit overflow in map_block_for_writepage()

I now see another overflow in reiserfs that should lead to data corruptions
with files that are bigger than 4G under certain circumstances when using
mmap.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>