x86, UV: Fix the effect of extra bits in the hub nodeid register

UV systems can be partitioned into multiple independent SSIs.
Large partitioned systems may have extra bits in the node_id
register. These bits are used when the total memory on all SSIs
exceeds 16TB.  These extra bits need to be ignored when
calculating x2apic_extra_bits.

Signed-off-by: Jack Steiner <steiner@sgi.com>
LKML-Reference: <20101130195926.972776133@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>