Normally the size is chosen as a multiple of the chunk size, but if the size
is explicitly chosen, it might not be.  So we force it.
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>