If raid1 decides it needs to resync it will do so even if there is only one
working device.  This is pointless.

With this patch we abort resync if there is nowhere to write to.
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>