Extend import_progress kstat with a notes field

Detail the import progress of log spacemaps as they can take a very
long time. Also grab the spa_note() messages to, as they provide
insight into what is happening

Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Closes #15539