[PATCH] init: fix -Wmissing-variable-declarations clang warning

[Justin Stitt <justinstitt@google.com>]

When building x86/defconfig with Clang-18 I encounter the following warning:
| init/main.c:189:13: warning: no previous extern declaration for non-static variable 'envp_init' [-Wmissing-variable-declarations]
|   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
| init/main.c:189:7: note: declare 'static' if the variable is not intended to be used outside of this translation unit
|   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };

Move the extern declaration to a header file so that the compiler can find it
when compiling init/main.c.

Add `const` qualifier to extern declaration so that the types match and
we follow the One-Definition Rule (ODR).

Link: https://github.com/ClangBuiltLinux/linux/issues/1920
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.

There's a previous RFC [1] wherein the kernel test robot reported some
build errors which lead me to creating this patch which both fixes the
errors as well as the -Wmissing-variable-declarations warning.

[1]: https://lore.kernel.org/all/20230829-missingvardecl-init-main-c-v1-1-ddf0f1a71215@google.com/

Also, get_maintainer.pl had a hard time finding folks to Cc here, not sure why.
---
 include/linux/initrd.h  | 2 ++
 init/do_mounts_initrd.c | 1 -
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/initrd.h b/include/linux/initrd.h
index f1a1f4c92ded..0114f1acbb45 100644
--- a/include/linux/initrd.h
+++ b/include/linux/initrd.h
@@ -34,4 +34,6 @@ extern unsigned long __initramfs_size;
 
 void console_on_rootfs(void);
 
+extern const char *envp_init[];
+
 #endif /* __LINUX_INITRD_H */
diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
index 425f4bcf4b77..b35c1b0babc2 100644
--- a/init/do_mounts_initrd.c
+++ b/init/do_mounts_initrd.c
@@ -87,7 +87,6 @@ static void __init handle_initrd(char *root_device_name)
 {
 	struct subprocess_info *info;
 	static char *argv[] = { "linuxrc", NULL, };
-	extern char *envp_init[];
 	int error;
 
 	pr_warn("using deprecated initrd support, will be removed in 2021.\n");

---
base-commit: 706a741595047797872e669b3101429ab8d378ef
change-id: 20230830-missingvardecl2-init-main-c-93dc1013ff8a

Best regards,
--
Justin Stitt <justinstitt@google.com>