x86/kprobes: Convert to insn_decode()

[ Upstream commit 77e768ec

 ]

Simplify code, improve decoding error checking.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lkml.kernel.org/r/20210304174237.31945-12-bp@alien8.de
Stable-dep-of: 63dc6325

 ("x86/kprobes: Fix optprobe optimization check with CONFIG_RETHUNK")
Signed-off-by: Sasha Levin <sashal@kernel.org>