FreeBSD: unregister mountroot eventhandler on unload

Otherwise if zfs is unloaded and reroot is being used it trips over a
stale pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: Rubicon Communications, LLC ("Netgate")
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #16242