drbd: take error path in drbd_adm_down if interrupted by signal

drbd_adm_down() does adm_detach(), which can fail with various error
codes, or be interrupted by a signal.

The interrupted by signal case was not properly handled,
leading to
	block drbd0: ASSERT( mdev->state.disk == D_DISKLESS &&
	                     mdev->state.conn == C_STANDALONE ) in drbd/drbd_worker.c
and further to destroying objects while still in use, and resulting crashes.

Detect the interruption, and take the error path out.

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