b=21574 more pinger fixes

b=21574 more pinger fixes

i=oleg
i=andrew

- ptlrpc_update_next_ping(): don't postpone next ping when "soon"
  is set and a ping request is already scheduled before the new
  deadline.
- It is usually fine to extend the deadline for the next ping
  since we are granted that the pinger will wake up before
  this new deadline and update his timer.
  However, the purpose of ptlrpc_pinger_commit_expected() is to
  schedule ping earlier. To support this, i've changed
  ptlrpc_update_next_ping() to wake up the pinger if the new
  ping deadline is before the pinger is supposed to wake up.