spa: ZIO_TASKQ_ISSUE: Use symbolic priority
This allows to change the meaning of priority differences in FreeBSD
without requiring code changes in ZFS.
This upstreams commit fd141584cf89d7d2 from FreeBSD src.
Sponsored-by: The FreeBSD Foundation
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Olivier Certner <olce@FreeBSD.org>
Closes #17489