Revert "x86/alternative: Make custom return thunk unconditional"

This reverts commit 08f7cfd4.

Revert the backport of upstream commit:

  095b8303 ("x86/alternative: Make custom return thunk unconditional")

in order to backport the full version now that

  770ae1b7 ("x86/returnthunk: Allow different return thunks")

has been backported.

Revert it here so that the build breakage is kept at minimum.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/include/asm/nospec-branch.h

@@ -190,11 +190,7 @@
 	_ASM_PTR " 999b\n\t"					\
 	".popsection\n\t"
 
-#ifdef CONFIG_RETHUNK
 extern void __x86_return_thunk(void);
-#else
-static inline void __x86_return_thunk(void) {}
-#endif
 
 extern void retbleed_return_thunk(void);
 extern void srso_return_thunk(void);

arch/x86/kernel/cpu/bugs.c

@@ -61,10 +61,6 @@ EXPORT_SYMBOL_GPL(x86_pred_cmd);
 
 static DEFINE_MUTEX(spec_ctrl_mutex);
 
-#ifdef CONFIG_CALL_THUNKS
-void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
-#endif
-
 /* Update SPEC_CTRL MSR and its cached copy unconditionally */
 static void update_spec_ctrl(u64 val)
 {