net: add EXPORT_INDIRECT_CALLABLE wrapper

When a static function is annotated with INDIRECT_CALLABLE_SCOPE and
CONFIG_RETPOLINE is set, the static keyword is removed. Sometimes the
function needs to be exported but EXPORT_SYMBOL can't be used because if
CONFIG_RETPOLINE is not set, we will attempt to export a static symbol.

This patch introduces a new indirect call wrapper:
EXPORT_INDIRECT_CALLABLE. This basically does EXPORT_SYMBOL when
CONFIG_RETPOLINE is set, but does nothing when it's not.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Brian Vazquez <brianvv@google.com>
Link: https://lore.kernel.org/r/20210204181839.558951-1-brianvv@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/indirect_call_wrapper.h

@@ -36,6 +36,7 @@
 
 #define INDIRECT_CALLABLE_DECLARE(f)	f
 #define INDIRECT_CALLABLE_SCOPE
+#define EXPORT_INDIRECT_CALLABLE(f)	EXPORT_SYMBOL(f)
 
 #else
 #define INDIRECT_CALL_1(f, f1, ...) f(__VA_ARGS__)
@@ -44,6 +45,7 @@
 #define INDIRECT_CALL_4(f, f4, f3, f2, f1, ...) f(__VA_ARGS__)
 #define INDIRECT_CALLABLE_DECLARE(f)
 #define INDIRECT_CALLABLE_SCOPE		static
+#define EXPORT_INDIRECT_CALLABLE(f)
 #endif
 
 /*