linux/abd_os: remove kernel version check for compound page support

All kernels we support have compound pages that work the way we would
like. However, this code is new and this knowledge was hard won, so I'd
like to leave the description and option there for a little while, even
if it can only be disabled with a recompile.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16545