objtool: Fix bytes check of lea's rex_prefix

For the "lea %(rsp), %rbp" case, we check if there is a rex_prefix.
But we check 'bytes' which is insn_byte_t[4] in rex_prefix (insn_field
structure). Therefore, the check is always true.

Instead, check 'nbytes' which is the right one.

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/20161205105551.25917-1-jslaby@suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: Jiri Slaby

tools/objtool/arch/x86/decode.c

@@ -99,7 +99,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 		break;
 
 	case 0x8d:
-		if (insn.rex_prefix.bytes &&
+		if (insn.rex_prefix.nbytes &&
 		    insn.rex_prefix.bytes[0] == 0x48 &&
 		    insn.modrm.nbytes && insn.modrm.bytes[0] == 0x2c &&
 		    insn.sib.nbytes && insn.sib.bytes[0] == 0x24)