objtool: Fix objtool fallthrough detection with function padding

When GCC adds NOP padding between functions, those NOPs aren't
associated with a function symbol, which breaks objtool's detection of a
function falling through to another function. Instead it shows
confusing errors like:

drivers/mtd/chips/cfi_util.o: warning: objtool: cfi_qry_mode_on()+0x8b: return with modified stack frame
drivers/mtd/chips/cfi_util.o: warning: objtool: cfi_qry_mode_on()+0x0: stack state mismatch: cfa1=-4-32 cfa2=7+8
drivers/mtd/chips/cfi_cmdset_0002.o: warning: objtool: fixup_use_fwh_lock()+0x8: unknown stack-related register move
drivers/mtd/chips/cfi_cmdset_0002.o: warning: objtool: fixup_use_fwh_lock()+0x0: stack state mismatch: cfa1=6+16 cfa2=7+8
drivers/mtd/chips/cfi_cmdset_0002.o: warning: objtool: do_otp_write()+0xa: unsupported stack pointer realignment
drivers/mtd/chips/cfi_cmdset_0002.o: warning: objtool: do_otp_write()+0x0: stack state mismatch: cfa1=-4-40 cfa2=7+8

Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

tools/objtool/check.c

···
 	while (1) {
 		next_insn = next_insn_same_sec(file, insn);
 
-		if (file->c_file && insn->func) {
-			if (func && func != insn->func) {
-				WARN("%s() falls through to next function %s()",
-				     func->name, insn->func->name);
-				return 1;
-			}
+
+		if (file->c_file && func && insn->func && func != insn->func) {
+			WARN("%s() falls through to next function %s()",
+			     func->name, insn->func->name);
+			return 1;
 		}
 
-		func = insn->func;
+		if (insn->func)
+			func = insn->func;
 
 		if (func && insn->ignore) {
 			WARN_FUNC("BUG: why am I validating an ignored function?",