x86/mmiotrace: Use cpumask_available() for cpumask_var_t variables

When building with Clang + -Wtautological-compare and
CONFIG_CPUMASK_OFFSTACK unset:

  arch/x86/mm/mmio-mod.c:375:6: warning: comparison of array 'downed_cpus'
  equal to a null pointer is always false [-Wtautological-pointer-compare]
          if (downed_cpus == NULL &&
              ^~~~~~~~~~~    ~~~~
  arch/x86/mm/mmio-mod.c:405:6: warning: comparison of array 'downed_cpus'
  equal to a null pointer is always false [-Wtautological-pointer-compare]
          if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
              ^~~~~~~~~~~    ~~~~
  2 warnings generated.

Commit

  f7e30f01 ("cpumask: Add helper cpumask_available()")

added cpumask_available() to fix warnings of this nature. Use that here
so that clang does not warn regardless of CONFIG_CPUMASK_OFFSTACK's
value.
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/982
Link: https://lkml.kernel.org/r/20200408205323.44490-1-natechancellor@gmail.com

arch/x86/mm/mmio-mod.c

@@ -372,7 +372,7 @@ static void enter_uniprocessor(void)
 	int cpu;
 	int err;
 
-	if (downed_cpus == NULL &&
+	if (!cpumask_available(downed_cpus) &&
 	    !alloc_cpumask_var(&downed_cpus, GFP_KERNEL)) {
 		pr_notice("Failed to allocate mask\n");
 		goto out;
@@ -402,7 +402,7 @@ static void leave_uniprocessor(void)
 	int cpu;
 	int err;
 
-	if (downed_cpus == NULL || cpumask_weight(downed_cpus) == 0)
+	if (!cpumask_available(downed_cpus) || cpumask_weight(downed_cpus) == 0)
 		return;
 	pr_notice("Re-enabling CPUs...\n");
 	for_each_cpu(cpu, downed_cpus) {