ch_mutex() was introduced with a mechanical conversion, but as we now have
correct locking we can remove it altogether.

Link: https://lore.kernel.org/r/20200213153207.123357-4-hare@suse.deReviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>