x86/sev-es: Use new for_each_insn_prefix() macro to loop over prefixes bytes

Since insn.prefixes.nbytes can be bigger than the size of
insn.prefixes.bytes[] when a prefix is repeated, the proper
check must be:

  insn.prefixes.bytes[i] != 0 and i < 4

instead of using insn.prefixes.nbytes. Use the new
for_each_insn_prefix() macro which does it correctly.

Debugged by Kees Cook <keescook@chromium.org>.

 [ bp: Massage commit message. ]

Fixes: 25189d08 ("x86/sev-es: Add support for handling IOIO exceptions")
Reported-by:  <syzbot+9b64b619f10f19d19a7c@syzkaller.appspotmail.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/160697106089.3146288.2052422845039649176.stgit@devnote2