Remove avl_size field from struct avl_tree

This field is used only by illumos mdb. On other platforms it only
increases the struct size from 32 to 40 bytes. For struct vdev_queue
including 13 instances of avl_tree_t size means active cache lines.

Keep the padding in user-space for now to not break the ABI.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12290