FreeBSD: switch teardown lock to rms

This deserializes otherwise non-contending operations.

The previous scheme of using 17 locks hashed by curthread runs into
conflicts very quickly. Check the pull request for sample results.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #11153