x86, UV: Fix and clean up bau code to use uv_gpa_to_pnode()

Create an inline function to extract the pnode from a global
physical address and then convert the broadcast assist unit to
use the newly created uv_gpa_to_pnode function.

The open-coded code was wrong as well - it might explain a
few of our unexplained bau hangs.

Signed-off-by: Robin Holt <holt@sgi.com>
Acked-by: Cliff Whickman <cpw@sgi.com>
Cc: linux-mm@kvack.org
Cc: Jack Steiner <steiner@sgi.com>
LKML-Reference: <20091016112920.GZ8903@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>