init.h: discard exitcall symbols early

Any __exitcall() and built-in module_exit() handler is marked as __used,
which leads to the code being included in the object file and later
discarded at link time.

As far as I can tell, this was originally added at the same time as
initcalls were marked the same way, to prevent them from getting dropped
with gcc-3.4, but it was never actaully necessary to keep exit functions
around.

Mark them as __maybe_unused instead, which lets the compiler treat the
exitcalls as entirely unused, and make better decisions about dropping
specializing static functions called from these.

Link: https://lore.kernel.org/all/acruxMNdnUlyRHiy@google.com/
Link: https://lore.kernel.org/20260331142846.3187706-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Schier <nsc@kernel.org>
Cc: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/init.h

@@ -47,7 +47,7 @@
 #define __initdata	__section(".init.data")
 #define __initconst	__section(".init.rodata")
 #define __exitdata	__section(".exit.data")
-#define __exit_call	__used __section(".exitcall.exit")
+#define __exit_call	__maybe_unused __section(".exitcall.exit")
 
 /*
  * modpost check for section mismatches during the kernel build.