Fix use-after-free in btree code

Coverty static analysis found these.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #10989
Closes #13861

module/zfs/btree.c

@@ -1608,8 +1608,8 @@ zfs_btree_remove_from_node(zfs_btree_t *tree, zfs_btree_core_t *node,
 	zfs_btree_poison_node_at(tree, keep_hdr, keep_hdr->bth_count, 1);
 
 	new_rm_hdr->bth_count = 0;
-	zfs_btree_node_destroy(tree, new_rm_hdr);
 	zfs_btree_remove_from_node(tree, parent, new_rm_hdr);
+	zfs_btree_node_destroy(tree, new_rm_hdr);
 }
 
 /* Remove the element at the specific location. */
@@ -1817,10 +1817,10 @@ zfs_btree_remove_idx(zfs_btree_t *tree, zfs_btree_index_t *where)
 
 	/* Move our elements to the left neighbor. */
 	bt_transfer_leaf(tree, rm, 0, rm_hdr->bth_count, keep, k_count + 1);
-	zfs_btree_node_destroy(tree, rm_hdr);
 
 	/* Remove the emptied node from the parent. */
 	zfs_btree_remove_from_node(tree, parent, rm_hdr);
+	zfs_btree_node_destroy(tree, rm_hdr);
 	zfs_btree_verify(tree);
 }