Do not deadlock in drbd_disconnect() [bugz 258]

When there are many blocks on the fly (ua), and the AL gets into "starving"
mode (random IO, scattered all over the device), and the connections gets
interrupted, the receiver thread deadlocks in the drbd_disconnect() code path.

Affected are only nodes in Primary role.

The bug triggers most likely on system that mirror over "long distances"

Regression introduced shortly before 8.3.3
with git commit 31e0f1250f174ac1ee317f360943a0159e19edc8

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