fix in-kernel configuration serialization

this is uncritical, as we still also serialize in userland,
but to correctly serialize on the CONFIG_PENDING bit,
it must be wait_event(state_wait, \!test_and_set_bit)

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>