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

include/sys/avl_impl.h

@@ -147,7 +147,9 @@ struct avl_tree {
 	int (*avl_compar)(const void *, const void *);
 	size_t avl_offset;		/* offsetof(type, avl_link_t field) */
 	ulong_t avl_numnodes;		/* number of nodes in the tree */
-	size_t avl_size;		/* sizeof user type struct */
+#ifndef _KERNEL
+	size_t avl_pad;			/* For backwards ABI compatibility. */
+#endif
 };