multipath: do not call tur in sync mode if pthread_cancel fails

When pthread_cancel fails the thread is stuck, most likely
during I/O submission. So it would be pointless to call the
tur checker in sync mode here, as this would be stuck, too.
Hence we should rather return 'PATH_TIMEOUT' and hope the
situation resolves itself over time.

References: bnc#832796

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