workqueue: Refactor worker ID formatting and make wq_worker_comm() use full ID string

Currently, worker ID formatting is open coded in create_worker(),
init_rescuer() and worker_thread() (for %WORKER_DIE case). The formatted ID
is saved into task->comm and wq_worker_comm() uses it as the base name to
append extra information to when generating the name to be shown to
userspace.

However, TASK_COMM_LEN is only 16 leading to badly truncated names for
rescuers. For example, the rescuer for the inet_frag_wq workqueue becomes:

  $ ps -ef | grep '[k]worker/R-inet'
  root         483       2  0 Apr26 ?        00:00:00 [kworker/R-inet_]

Even for non-rescue workers, it's easy to run over 15 characters on
moderately large machines.

Fit it by consolidating worker ID formatting into a new helper
format_worker_id() and calling it from wq_worker_comm() to obtain the
untruncated worker ID string.

  $ ps -ef | grep '[k]worker/R-inet'
  root          60       2  0 12:10 ?        00:00:00 [kworker/R-inet_frag_wq]

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Jan Engelhardt <jengelh@inai.de>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>