x86/SMP: CPU0's scratch mask is needed earlier

x86/SMP: CPU0's scratch mask is needed earlier

When putting together commit 3b61726458 ("x86: introduce and use
scratch CPU mask") I failed to remember that AMD IOMMU setups needs the
scratch mask prior to smp_prepare_cpus() having run. Use a static mask
for the boot CPU instead.

Note that the definition of scratch_cpu0mask could also be put inside a
"NR_CPUS > 2 * BITS_PER_LONG" conditional, but it seems preferable to
me to carry the extra variable in all cases and avoid the #ifdef-ary.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>