drbd: fix race when forcefully disconnecting

If a forced disconnect hits a restarting receiver right after it passed
its final "if (C_DISCONNECTING)" test in drbdd_init(), but before it was
actually restarted by drbd_thread_setup, we could be left with a
connection stuck in C_DISCONNECTING, never reaching C_STANDALONE,
which would be necessary to take it down or reconfigure it.

Move the last cleanup into w_after_conn_state_ch(), and do an additional
state change request in conn_try_disconnect(), just in case.

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