init: verify that function is initcall_t at compile-time

In the spirit of making it hard to misuse an interface, add a
compile-time assertion in the CONFIG_HAVE_ARCH_PREL32_RELOCATIONS case
to verify the initcall function matches initcall_t, because the inline
asm bypasses any type-checking the compiler would otherwise do. This
will help developers catch incorrect API use in all configurations.

A recent example of this is:
https://lkml.kernel.org/r/20210514140015.2944744-1-arnd@kernel.org

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210521072610.2880286-1-elver@google.com

include/linux/init.h

@@ -242,7 +242,8 @@ extern bool initcall_debug;
 	asm(".section	\"" __sec "\", \"a\"		\n"	\
 	    __stringify(__name) ":			\n"	\
 	    ".long	" __stringify(__stub) " - .	\n"	\
-	    ".previous					\n");
+	    ".previous					\n");	\
+	static_assert(__same_type(initcall_t, &fn));
 #else
 #define ____define_initcall(fn, __unused, __name, __sec)	\
 	static initcall_t __name __used 			\