objtool: Fix function fallthrough detection

When a function falls through to the next function due to a compiler
bug, objtool prints some obscure warnings.  For example:

  drivers/regulator/core.o: warning: objtool: regulator_count_voltages()+0x95: return with modified stack frame
  drivers/regulator/core.o: warning: objtool: regulator_count_voltages()+0x0: stack state mismatch: cfa1=7+32 cfa2=7+8

Instead it should be printing:

  drivers/regulator/core.o: warning: objtool: regulator_supply_is_couple() falls through to next function regulator_count_voltages()

This used to work, but was broken by the following commit:

  13810435b9 ("objtool: Support GCC 8's cold subfunctions")

The padding nops at the end of a function aren't actually part of the
function, as defined by the symbol table.  So the 'func' variable in
validate_branch() is getting cleared to NULL when a padding nop is
encountered, breaking the fallthrough detection.

If the current instruction doesn't have a function associated with it,
just consider it to be part of the previously detected function by not
overwriting the previous value of 'func'.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Fixes: 13810435b9 ("objtool: Support GCC 8's cold subfunctions")
Link: http://lkml.kernel.org/r/546d143820cd08a46624ae8440d093dd6c902cae.1557766718.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/check.c

@@ -1959,7 +1959,8 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			return 1;
 		}
 
-		func = insn->func ? insn->func->pfunc : NULL;
+		if (insn->func)
+			func = insn->func->pfunc;
 
 		if (func && insn->ignore) {
 			WARN_FUNC("BUG: why am I validating an ignored function?",