sysctl: add register_sysctl() dummy helper

register_sysctl() has been around for five years with commit
fea478d ("sysctl: Add register_sysctl for normal sysctl users") but
now that arm64 started using it, I ran into a compile error:

  arch/arm64/kernel/armv8_deprecated.c: In function 'register_insn_emulation_sysctl':
  arch/arm64/kernel/armv8_deprecated.c:257:2: error: implicit declaration of function 'register_sysctl'

This adds a inline function like we already have for
register_sysctl_paths() and register_sysctl_table().

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 38b9aeb ("arm64: Port deprecated instruction emulation to new sysctl interface")
Signed-off-by: Arnd Bergmann <[email protected]>
Reviewed-by: Dave Martin <[email protected]>
Acked-by: Kees Cook <[email protected]>
Acked-by: Will Deacon <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: "Luis R. Rodriguez" <[email protected]>
Cc: Alex Benne <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/sysctl.h

@@ -213,6 +213,11 @@ static inline struct ctl_table_header *register_sysctl_paths(
 	return NULL;
 }
 
+static inline struct ctl_table_header *register_sysctl(const char *path, struct ctl_table *table)
+{
+	return NULL;
+}
+
 static inline void unregister_sysctl_table(struct ctl_table_header * table)
 {
 }