[Crash-utility] HEADS UP: upstream kernel pidhash removal

[Dave Anderson]

FYI, if you're running with bleeding edge upstream kernels, this
recent commit destroys a fundamental underpinning required by the
crash utility:

  commit e8cfbc245e24887e3c30235f71e9e9405e0cfc39
  Author: Gargi Sharma <gs051095 at gmail.com>
  Date:   Fri Nov 17 15:30:34 2017 -0800

    pid: remove pidhash
    
    pidhash is no longer required as all the information can be looked up
    from idr tree.  nr_hashed represented the number of pids that had been
    hashed.  Since, nr_hashed and PIDNS_HASH_ADDING are no longer relevant,
    it has been renamed to pid_allocated and PIDNS_ADDING respectively.
    
I'm not sure how the failure will manifest itself, but it certainly will
will fail early on during session initialization.  When I saw the original 
patch postings, I started tinkering with the creation of yet another
function to plug into "tt->refresh_task_table" for the new IDR/radix-tree
scheme, but until I see an actual live kernel and/or vmcore with the patch
above, I've held off on further work on it.

In the meantime, you can use the "crash --active" command line option,
which restricts the task list to those that are registered as the current
task in each per-cpu runqueue.

Dave