work queue: make trace related code compile aware

This patches removes trace code if trace is not enabled. Based on Hitoshi Mitake
original patch:

- lib, sheep: exclude stuff for tracing when it is not enabled

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
work: improve namings and comments

This patch removes confusing names in work.c:
1. before: wi->nr_workers, after: wi->nr_queued_work
2. before: struct worker_info, after: struct wq_info

Also fixes obsolete comments.

[ rename wq_info_siblings as 'list' - Yuan ]
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>

Conflicts:
	lib/work.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>