Fix objtool: missing int3 after ret warning

Resolve straight-line speculation warnings reported by objtool
for x86_64 assembly on Linux when CONFIG_SLS is set. See the
following LWN article for the complete details.

https://lwn.net/Articles/877845/

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575