cpumask: Add initialiser to use cleanup helpers

Now we can simplify a code that allocates cpumasks for local needs.

Automatic variables have to be initialized at declaration, or at least
before any possibility for the logic to return, so that compiler
wouldn't try to call an associate destructor function on a random stack
number.

Because cpumask_var_t, depending on the CPUMASK_OFFSTACK config, is
either a pointer or an array, we have to have a macro for initialization.

So define a CPUMASK_VAR_NULL macro, which allows to init struct cpumask
pointer with NULL when CPUMASK_OFFSTACK is enabled, and effectively a
no-op when CPUMASK_OFFSTACK is disabled (initialisation optimised out
with -O2).

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251120145653.296659-7-gmonaco@redhat.com

Author: YuryNorov

include/linux/cpumask.h

@@ -1005,6 +1005,7 @@ static __always_inline unsigned int cpumask_size(void)
 
 #define this_cpu_cpumask_var_ptr(x)	this_cpu_read(x)
 #define __cpumask_var_read_mostly	__read_mostly
+#define CPUMASK_VAR_NULL		NULL
 
 bool alloc_cpumask_var_node(cpumask_var_t *mask, gfp_t flags, int node);
 
@@ -1051,6 +1052,7 @@ static __always_inline bool cpumask_available(cpumask_var_t mask)
 
 #define this_cpu_cpumask_var_ptr(x) this_cpu_ptr(x)
 #define __cpumask_var_read_mostly
+#define CPUMASK_VAR_NULL {}
 
 static __always_inline bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
 {