Asynchronous mode for tur checker

Asynchronous mode for tur checker

When a path is going down I/O will stall until the underlying
device becomes unblocked. This will stall the entire checker loop
and cause severe delays.
This patch implements an asynchronous mode for the 'TUR' checker,
whereby the actual ioctl is executed in its own thread. The
checkerloop then just examines the state of this separate thread
and can continue even if the ioctl is blocked.

Signed-off-by: Hannes Reinecke <hare@suse.de>