md: remove mddev_lock from rdev_attr_show()

No rdev attributes need locking for 'show', though
state_show() might benefit from ensuring it sees a
consistent set of flags.

None even use rdev->mddev, so testing for it isn't really
needed and it certainly doesn't need to be held constant.

So improve state_show() and remove the locking.

Signed-off-by: NeilBrown <neilb@suse.de>