ftrace,kcfi: Define ftrace_stub_graph conditionally

When CONFIG_FUNCTION_GRAPH_TRACER is disabled, __kcfi_typeid_ftrace_stub_graph
is missing, causing a link failure:

 ld.lld: error: undefined symbol: __kcfi_typeid_ftrace_stub_graph
 referenced by arch/x86/kernel/ftrace_64.o:(__cfi_ftrace_stub_graph) in archive vmlinux.a

Mark the reference to it as conditional on the same symbol, as
is done on arm64.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Peter Zijlstra <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Fixes: 883bbbf ("ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()")
See-also: 2598ac6 ("arm64: ftrace: Define ftrace_stub_graph only with FUNCTION_GRAPH_TRACER")
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: arndb

arch/x86/kernel/ftrace_64.S

@@ -136,10 +136,12 @@ SYM_TYPED_FUNC_START(ftrace_stub)
 	RET
 SYM_FUNC_END(ftrace_stub)
 
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
 SYM_TYPED_FUNC_START(ftrace_stub_graph)
 	CALL_DEPTH_ACCOUNT
 	RET
 SYM_FUNC_END(ftrace_stub_graph)
+#endif
 
 #ifdef CONFIG_DYNAMIC_FTRACE