x86/numa: Use cpumask_available instead of hardcoded NULL check

GCC-12 started triggering a new warning:

  arch/x86/mm/numa.c: In function ‘cpumask_of_node’:
  arch/x86/mm/numa.c:916:39: warning: the comparison will always evaluate as ‘false’ for the address of ‘node_to_cpumask_map’ will never be NULL [-Waddress]
    916 |         if (node_to_cpumask_map[node] == NULL) {
        |                                       ^~

node_to_cpumask_map is of type cpumask_var_t[].

When CONFIG_CPUMASK_OFFSTACK is set, cpumask_var_t is typedef'd to a
pointer for dynamic allocation, else to an array of one element. The
"wicked game" can be checked on line 700 of include/linux/cpumask.h.

The original code in debug_cpumask_set_cpu() and cpumask_of_node() were
probably written by the original authors with CONFIG_CPUMASK_OFFSTACK=y
(i.e. dynamic allocation) in mind, checking if the cpumask was available
via a direct NULL check.

When CONFIG_CPUMASK_OFFSTACK is not set, GCC gives the above warning
while compiling the kernel.

Fix that by using cpumask_available(), which does the NULL check when
CONFIG_CPUMASK_OFFSTACK is set, otherwise returns true. Use it wherever
such checks are made.

Conditional definitions of cpumask_available() can be found along with
the definition of cpumask_var_t. Check the cpumask.h reference mentioned
above.

Fixes: c032ef60 ("cpumask: convert node_to_cpumask_map[] to cpumask_var_t")
Fixes: de2d9445 ("x86: Unify node_to_cpumask_map handling between 32 and 64bit")
Signed-off-by: Siddh Raman Pant <code@siddh.me>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20220731160913.632092-1-code@siddh.me

arch/x86/mm/numa.c

@@ -867,7 +867,7 @@ void debug_cpumask_set_cpu(int cpu, int node, bool enable)
 		return;
 	}
 	mask = node_to_cpumask_map[node];
-	if (!mask) {
+	if (!cpumask_available(mask)) {
 		pr_err("node_to_cpumask_map[%i] NULL\n", node);
 		dump_stack();
 		return;
@@ -913,7 +913,7 @@ const struct cpumask *cpumask_of_node(int node)
 		dump_stack();
 		return cpu_none_mask;
 	}
-	if (node_to_cpumask_map[node] == NULL) {
+	if (!cpumask_available(node_to_cpumask_map[node])) {
 		printk(KERN_WARNING
 			"cpumask_of_node(%d): no node_to_cpumask_map!\n",
 			node);