md: handle writes to broken raid10 arrays gracefully

When writing to a broken array, raid10 currently happily emits empty bio
lists.  IOW, the master bio will never be completed, sending writers to
UNINTERRUPTIBLE_SLEEP forever.

Signed-off-by: Arne Redlich <agr@powerkom-dd.de>
Acked-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>