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/20171106133700.558647-1-arnd@arndb.de
Fixes: 38b9aeb ("arm64: Port deprecated instruction emulation to new sysctl interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Alex Benne <alex.bennee@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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)
 {
 }