LU-6142 ldlm: tidy list walking in ldlm_flock()

LU-6142 ldlm: tidy list walking in ldlm_flock()

Use list_for_each_entry variants to
avoid the explicit list_entry() calls.
This allows us to use list_for_each_entry_safe_from()
instread of adding a local list-walking macro.

Also improve some comments so that it is more obvious
that the locks are sorted per-owner and that we need
to find the insertion point.

Linux-commit: 3ac5a67 ("staging: lustre: ldlm: tidy list walking in ldlm_flock()")

Change-Id: Ie9a756a898a9c58db1b4f446694603a4efa37352
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49358
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>