LU-6142 ldlm: use list_first_entry in ldlm_lock

LU-6142 ldlm: use list_first_entry in ldlm_lock

This make the code (slightly) more readable.

Linux-commit: ef7e70a ("staging: lustre: ldlm: use list_first_entry in ldlm_lock")

Change-Id: If9789fef1dec55d08dec25819aaf5152946819c5
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/+/49359
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>