objtool: Fix IRET's opcode

The IRET opcode is 0xcf according to the Intel manual and also to objdump of my
vmlinux:

    1ea8:       48 cf                   iretq

Fix the opcode in arch_decode_instruction().

The previous value (0xc5) seems to correspond to LDS.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170118132921.19319-1-jslaby@suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/arch/x86/decode.c

@@ -150,9 +150,9 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 		*type = INSN_RETURN;
 		break;
 
-	case 0xc5: /* iret */
 	case 0xca: /* retf */
 	case 0xcb: /* retf */
+	case 0xcf: /* iret */
 		*type = INSN_CONTEXT_SWITCH;
 		break;