workqueue: Make show_pwq() use run-length encoding

The show_pwq() function dumps out a pool_workqueue structure's activity,
including the pending work-queue handlers:

 Showing busy workqueues and worker pools:
 workqueue events: flags=0x0
   pwq 0: cpus=0 node=0 flags=0x1 nice=0 active=10/256 refcnt=11
     in-flight: 7:test_work_func, 64:test_work_func, 249:test_work_func
     pending: test_work_func, test_work_func, test_work_func1, test_work_func1, test_work_func1, test_work_func1, test_work_func1

When large systems are facing certain types of hang conditions, it is not
unusual for this "pending" list to contain runs of hundreds of identical
function names.  This "wall of text" is difficult to read, and worse yet,
it can be interleaved with other output such as stack traces.

Therefore, make show_pwq() use run-length encoding so that the above
printout instead looks like this:

 Showing busy workqueues and worker pools:
 workqueue events: flags=0x0
   pwq 0: cpus=0 node=0 flags=0x1 nice=0 active=10/256 refcnt=11
     in-flight: 7:test_work_func, 64:test_work_func, 249:test_work_func
     pending: 2*test_work_func, 5*test_work_func1

When no comma would be printed, including the WORK_STRUCT_LINKED case,
a new run is started unconditionally.

This output is more readable, places less stress on the hardware,
firmware, and software on the console-log path, and reduces interference
with other output.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>