The BAU on UV4 does not need to maintain the payload queue tail pointer. Do
not initialize the tail pointer MMR on UV4.

Note that write_payload_tail is not an abstracted BAU function since it is
an operation specific to pre-UV4 versions. Then we must switch on the UV
version to control its usage, for which we use uvhub_version rather than
is_uv*_hub because it is quicker/more concise.
Signed-off-by: Andrew Banman <abanman@sgi.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mike Travis <travis@sgi.com>
Acked-by: Dimitri Sivanich <sivanich@sgi.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: akpm@linux-foundation.org
Cc: rja@sgi.com
Link: http://lkml.kernel.org/r/1474474161-265604-10-git-send-email-abanman@sgi.comSigned-off-by: Ingo Molnar <mingo@kernel.org>