UV: NMI: insert this_cpu_read accessor function on uv_hub_nmi.

On SGI UV systems a 'power nmi' command from the CMC causes
all processors to drop into uv_handle_nmi().  With the 4.0
kernel this results in

    BUG: unable to handle kernel paging request

The bug is caused by the current code trying to use the PER_CPU
variable uv_cpu_nmi.hub without an appropriate accessor
function. That oversight occurred in

    commit e1632170 ("uv: Replace __get_cpu_var")
    Author: Christoph Lameter <cl@linux.com>
    Date:   Sun Aug 17 12:30:41 2014 -0500

This patch inserts this_cpu_read() in the uv_hub_nmi macro
restoring the intended functionality.
Signed-off-by: George Beshers <gbeshers@sgi.com>
Acked-by: Mike Travis <travis@sgi.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Cc: Hedi Berriche <hedi@sgi.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russ Anderson <rja@sgi.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>