Fix use-after-free in btree code

Coverty static analysis found these.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Neal Gompa <[email protected]>
Signed-off-by: Richard Yao <[email protected]>
Closes #10989
Closes #13861

Approved by:	so
Security:	FreeBSD-EN-22:21.zfs

(cherry picked from commit 8131a96544)

sys/contrib/openzfs/module/zfs/btree.c

@@ -1536,8 +1536,8 @@ 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);
 
 	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. */
@@ -1769,6 +1769,7 @@ zfs_btree_remove_idx(zfs_btree_t *tree, zfs_btree_index_t *where)
 	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);
 }