include: move zio_priority_t into zfs.h

It's included so it's effectively already part of it, but it's not
always installed as a userspace header, making zfs.h effectively
useless. Might as well just combine it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Close #17066