md: Don't try to set an array to 'read-auto' if it is already in that state.

commit 80268ee9270ebe4847365a7426de91d179e870d0 upstream.

'read-auto' is a variant of 'readonly' which will switch to writable
on the first write attempt.

Calling do_md_stop to set the array readonly when it is already readonly
returns an error.  So make sure not to do that.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>