[IA64] Fix broken sn2 build

Revert the change made to arch/ia64/sn/kernel/setup.c by commit
204fba4a as it breaks the build.

Fixing the build the b94b0808 way
breaks xpc because genksyms then fails to generate an CRC for
per_cpu____sn_cnodeid_to_nasid because of limitations in the
generic genksyms code.

Signed-off-by: Hedi Berriche <hedi@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>