ia64: make cpu_callin_map non-volatile.

cpumask_test_cpu() doesn't take volatile, unlike the obsoleted
cpu_isset.  The only place ia64 really cares is the spin waiting for a
bit; udelay() is probably a barrier but insert barrier() to be sure.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>