[PATCH] md: fix BUG when raid10 rebuilds without enough drives

This shouldn't be a BUG.  We should cope.

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