module: LLVMLinux: Remove unused function warning from __param_check macro

This code makes a compile time type check that is optimized away. Clang
complains that it generates an unused function:

linux/kernel/panic.c:471:1: warning: unused function '__check_panic'
      [-Wunused-function]
core_param(panic, panic_timeout, int, 0644);
^
linux/moduleparam.h:283:2: note: expanded from macro
      'core_param'
        param_check_##type(name, &(var));                               \
        ^
<scratch space>:87:1: note: expanded from here
param_check_int
^
linux/moduleparam.h:369:34: note: expanded from macro
      'param_check_int'
#define param_check_int(name, p) __param_check(name, p, int)
                                 ^
linux/moduleparam.h:349:22: note: expanded from macro
      '__param_check'
        static inline type *__check_##name(void) { return(p); }
                            ^
<scratch space>:88:1: note: expanded from here
__check_panic

GCC won't complain for a static inline function but would if it was just
a static function.

Adding the unused attribute to the function declaration removes the warning.
Per request from Rusty Russell it is marked as __always_unused as the code
is meant to be optimized away.

This code works for both GCC and clang.
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/moduleparam.h

@@ -346,7 +346,7 @@ static inline void destroy_params(const struct kernel_param *params,
 /* The macros to do compile-time type checking stolen from Jakub
    Jelinek, who IIRC came up with this idea for the 2.4 module init code. */
 #define __param_check(name, p, type) \
-	static inline type *__check_##name(void) { return(p); }
+	static inline type __always_unused *__check_##name(void) { return(p); }
 
 extern struct kernel_param_ops param_ops_byte;
 extern int param_set_byte(const char *val, const struct kernel_param *kp);