LU-6142 lustre: obdclass: simplify cl_lock_fini()

LU-6142 lustre: obdclass: simplify cl_lock_fini()

Using list_first_entry_or_null() makes this (slightly)
simpler.

Linux-commit: 988b9ea9129bc24baf36ee421feb823285f234c4

Change-Id: Ic2fe2bb58b67781c8bc7b4e81cbf6b61dcaa56fb
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49736
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>