[PATCH] md: Calculate correct array size for raid10 in new offset mode

The size calculation made assumtion which the new offset mode didn't
follow.  This gets the size right in all cases.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>