objtool: Limit unreachable warnings to once per function

Unreachable instruction warnings are limited to once per object file.
That no longer makes sense for vmlinux validation, which might have
more unreachable instructions lurking in other places. Change it to
once per function.

Note this affects some other (much rarer) non-fatal warnings as well.
In general I think one-warning-per-function makes sense, as related
warnings can accumulate quickly and we want to eventually get back to
failing the build with -Werror anyway.

Reviewed-by: Miroslav Benes <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/check.c

···
 static int validate_reachable_instructions(struct objtool_file *file)
 {
 	struct instruction *insn;
+	int warnings = 0;
 
 	if (file->ignore_unreachables)
 		return 0;
···
 			continue;
 
 		WARN_INSN(insn, "unreachable instruction");
-		return 1;
+		warnings++;
 	}
 
-	return 0;
+	return warnings;
 }
 
 int check(struct objtool_file *file)

tools/objtool/include/objtool/elf.h

···
 	u8 return_thunk      : 1;
 	u8 fentry            : 1;
 	u8 profiling_func    : 1;
+	u8 warned	     : 1;
 	struct list_head pv_target;
 	struct list_head reloc_list;
 };

tools/objtool/include/objtool/warn.h

···
 
 #define WARN_INSN(insn, format, ...)					\
 ({									\
-	WARN_FUNC(format, insn->sec, insn->offset,  ##__VA_ARGS__);	\
+	struct instruction *_insn = (insn);				\
+	if (!_insn->sym || !_insn->sym->warned)				\
+		WARN_FUNC(format, _insn->sec, _insn->offset,		\
+			  ##__VA_ARGS__);				\
+	if (_insn->sym)							\
+		_insn->sym->warned = 1;					\
 })
 
 #define BT_FUNC(format, insn, ...)			\