x86/insn: Add @buf_len param to insn_init() kernel-doc comment

It wasn't documented so add it. No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lkml.kernel.org/r/20210304174237.31945-3-bp@alien8.de