[PATCH] md: fix raid10 assembly when too many devices are missing

If you try to assemble an array with too many missing devices, raid10 will now
reject the attempt, instead of allowing it.

Also check when hot-adding a drive and refuse the hot-add if the array is
beyond hope.

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