[PATCH] Fix raid0's attempt to divide by 64bit numbers

Apparently sector_div is only guaranteed to work with a 32bit divisor, even
on 64bit architectures.  So allow for this in raid0.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>