kasan: split kasan_*enabled() functions into a separate header

In an upcoming commit we are going to need to call
kasan_hw_tags_enabled() from arch/arm64/include/asm/mte.h. This
would create a circular dependency between headers if KASAN_GENERIC
or KASAN_SW_TAGS is enabled: linux/kasan.h -> linux/pgtable.h ->
asm/pgtable.h -> asm/mte.h -> linux/kasan.h. Break the cycle
by introducing a new header linux/kasan-enabled.h with the
kasan_*enabled() functions that can be included from asm/mte.h.

Link: https://linux-review.googlesource.com/id/I5b0d96c6ed0026fc790899e14d42b2fac6ab568eSigned-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/r/20220219012945.894950-1-pcc@google.comSigned-off-by: Will Deacon <will@kernel.org>

include/linux/kasan-enabled.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_KASAN_ENABLED_H
+#define _LINUX_KASAN_ENABLED_H
+
+#ifdef CONFIG_KASAN_HW_TAGS
+
+DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
+
+static __always_inline bool kasan_enabled(void)
+{
+	return static_branch_likely(&kasan_flag_enabled);
+}
+
+static inline bool kasan_hw_tags_enabled(void)
+{
+	return kasan_enabled();
+}
+
+#else /* CONFIG_KASAN_HW_TAGS */
+
+static inline bool kasan_enabled(void)
+{
+	return IS_ENABLED(CONFIG_KASAN);
+}
+
+static inline bool kasan_hw_tags_enabled(void)
+{
+	return false;
+}
+
+#endif /* CONFIG_KASAN_HW_TAGS */
+
+#endif /* LINUX_KASAN_ENABLED_H */

include/linux/kasan.h

@@ -3,6 +3,7 @@
 #define _LINUX_KASAN_H
 
 #include <linux/bug.h>
+#include <linux/kasan-enabled.h>
 #include <linux/kernel.h>
 #include <linux/static_key.h>
 #include <linux/types.h>
@@ -83,33 +84,11 @@ static inline void kasan_disable_current(void) {}
 
 #ifdef CONFIG_KASAN_HW_TAGS
 
-DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
-
-static __always_inline bool kasan_enabled(void)
-{
-	return static_branch_likely(&kasan_flag_enabled);
-}
-
-static inline bool kasan_hw_tags_enabled(void)
-{
-	return kasan_enabled();
-}
-
 void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags);
 void kasan_free_pages(struct page *page, unsigned int order);
 
 #else /* CONFIG_KASAN_HW_TAGS */
 
-static inline bool kasan_enabled(void)
-{
-	return IS_ENABLED(CONFIG_KASAN);
-}
-
-static inline bool kasan_hw_tags_enabled(void)
-{
-	return false;
-}
-
 static __always_inline void kasan_alloc_pages(struct page *page,
 					      unsigned int order, gfp_t flags)
 {