IB/hfi1: Allocate cpu mask on the heap to silence warning

If CONFIG_FRAME_WARN is small (1K) and CONFIG_NR_CPUS big
then a frame size warning is triggered during build.
Allocate the cpu mask dynamically to silence the warning.

Reviewed-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>