arm64: kasan: fix issues reported by sparse

Sparse reports some new issues introduced by the kasan patches:

  arch/arm64/mm/kasan_init.c:91:13: warning: no previous prototype for
  'kasan_early_init' [-Wmissing-prototypes] void __init kasan_early_init(void)
             ^
  arch/arm64/mm/kasan_init.c:91:13: warning: symbol 'kasan_early_init'
  was not declared. Should it be static? [sparse]

This patch resolves the problem by adding a prototype for
kasan_early_init and marking the function as asmlinkage, since it's only
called from head.S.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/kasan.h

@@ -5,6 +5,7 @@
 
 #ifdef CONFIG_KASAN
 
+#include <linux/linkage.h>
 #include <asm/memory.h>
 
 /*
@@ -27,6 +28,7 @@
 #define KASAN_SHADOW_OFFSET     (KASAN_SHADOW_END - (1ULL << (64 - 3)))
 
 void kasan_init(void);
+asmlinkage void kasan_early_init(void);
 
 #else
 static inline void kasan_init(void) { }