LU-6142 ldlm: use list_first_entry in ldlm_lockd.c

LU-6142 ldlm: use list_first_entry in ldlm_lockd.c

This is only a small simplification, but it makes the code
a little clearer.

Linux-commit: 7378caf4fe5198ce572654c926437fba12fb2255

Change-Id: Ie65049e12a1b1bbe448baefc38a6657d831e0670
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/+/49737
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>