Retain thread name when resuming a zthr

When created, a zthr is given a name to identify it by. This name is
lost when a cancelled zthr is resumed.

Retain the name of a zthr so it can be used when resuming.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10881