On enslave failure primary_slave can point to new_slave which is to be
freed, and the same applies to curr_active_slave. So check if this is
the case and clean up properly after err_detach because that's the first
error code path after they're set.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>