x86/coco: Replace 'static const cc_mask' with the newly introduced cc_get_mask() function

When extra warnings are enabled, the cc_mask definition in <asm/coco.h>
causes a build failure with GCC:

  arch/x86/include/asm/coco.h:28:18: error: 'cc_mask' defined but not used [-Werror=unused-const-variable=]
     28 | static const u64 cc_mask = 0;

Add a cc_get_mask() function mirroring cc_set_mask() for the one
user of the variable outside of the CoCo implementation.

Fixes: a0a8d15 ("x86/tdx: Preserve shared bit on mprotect()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250310131114.2635497-1-arnd@kernel.org

--
v2: use an inline helper instead of a __maybe_unused annotaiton.

Author: arndb

arch/x86/include/asm/coco.h

@@ -15,6 +15,11 @@ enum cc_vendor {
 extern enum cc_vendor cc_vendor;
 extern u64 cc_mask;
 
+static inline u64 cc_get_mask(void)
+{
+	return cc_mask;
+}
+
 static inline void cc_set_mask(u64 mask)
 {
 	RIP_REL_REF(cc_mask) = mask;
@@ -25,7 +30,10 @@ u64 cc_mkdec(u64 val);
 void cc_random_init(void);
 #else
 #define cc_vendor (CC_VENDOR_NONE)
-static const u64 cc_mask = 0;
+static inline u64 cc_get_mask(void)
+{
+	return 0;
+}
 
 static inline u64 cc_mkenc(u64 val)
 {

arch/x86/include/asm/pgtable_types.h

@@ -177,7 +177,7 @@ enum page_cache_mode {
 };
 #endif
 
-#define _PAGE_CC		(_AT(pteval_t, cc_mask))
+#define _PAGE_CC		(_AT(pteval_t, cc_get_mask()))
 #define _PAGE_ENC		(_AT(pteval_t, sme_me_mask))
 
 #define _PAGE_CACHE_MASK	(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)