The only access in md_seq_show that could suffer from races
not protected by ->lock is walking the rdev list.
This can receive sufficient protection from 'rcu'.

So use rdev_for_each_rcu() and get rid of mddev_lock().

Now reading /proc/mdstat will never block in md_seq_show.
Signed-off-by: NeilBrown <neilb@suse.de>