Make sure avl_tree.avl_pad is not in kernel module (#16280)

The commit b192a2c (Remove avl_size field from struct avl_tree) uses a
def _KERNEL to decide to include avl_pad or not, but this _KERNEL is
defined in sys/sysmacros.h. If avl.h and sysmacros.h are not included
in the right order, it can cause a headache when working on a zfs
related kernel module.

Add sysmacros.h in avl_impl.h to fix. sysmacros.h is also removed
from spa.h as it's reduntant.

Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Co-authored-by: Youzhong Yang <yyang@mathworks.com>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>