x86/uv: Work around UV2 BAU hangs

On SGI's UV2 the BAU (Broadcast Assist Unit) driver can hang
under a heavy load. To cure this:

- Disable the UV2 extended status mode (see UV2_EXT_SHFT), as
  this mode changes BAU behavior in more ways then just delivering
  an extra bit of status.  Revert status to just two meaningful bits,
  like UV1.

- Use no IPI-style resets on UV2.  Just give up the request for
  whatever the reason it failed and let it be accomplished with
  the legacy IPI method.

- Use no alternate sending descriptor (the former UV2 workaround
  bcp->using_desc and handle_uv2_busy() stuff).  Just disable the
  use of the BAU for a period of time in favor of the legacy IPI
  method when the h/w bug leaves a descriptor busy.

  -- new tunable: giveup_limit determines the threshold at which a hub is
     so plugged that it should do all requests with the legacy IPI method for a
     period of time
  -- generalize disable_for_congestion() (renamed disable_for_period()) for
     use whenever a hub should avoid using the BAU for a period of time

Also:

 - Fix find_another_by_swack(), which is part of the UV2 bug workaround

 - Correct and clarify the statistics (new stats s_overipilimit, s_giveuplimit,
   s_enters, s_ipifordisabled, s_plugged, s_congested)

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Link: http://lkml.kernel.org/r/20120622131459.GC31884@sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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