Linux 6.8 compat: replace MAX_ORDER define

MAX_ORDER has been renamed to MAX_PAGE_ORDER. Rather than just
redefining it, instead define our own name and set it consistently from
the start.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15805