fuse: move rb_erase() before tree_insert()

In fuse_writepage_end() the old writepages entry needs to be removed from
the rbtree before inserting the new one, otherwise tree_insert() would
fail.  This is a very rare codepath and no reproducer exists.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1586,7 +1586,6 @@ static void fuse_writepage_finish(struct fuse_conn *fc,
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	int i;
 
-	rb_erase(&wpa->writepages_entry, &fi->writepages);
 	for (i = 0; i < ap->num_pages; i++) {
 		dec_wb_stat(&bdi->wb, WB_WRITEBACK);
 		dec_node_page_state(ap->pages[i], NR_WRITEBACK_TEMP);
@@ -1637,6 +1636,7 @@ __acquires(fi->lock)
 
  out_free:
 	fi->writectr--;
+	rb_erase(&wpa->writepages_entry, &fi->writepages);
 	fuse_writepage_finish(fc, wpa);
 	spin_unlock(&fi->lock);
 
@@ -1714,6 +1714,7 @@ static void fuse_writepage_end(struct fuse_conn *fc, struct fuse_args *args,
 
 	mapping_set_error(inode->i_mapping, error);
 	spin_lock(&fi->lock);
+	rb_erase(&wpa->writepages_entry, &fi->writepages);
 	while (wpa->next) {
 		struct fuse_conn *fc = get_fuse_conn(inode);
 		struct fuse_write_in *inarg = &wpa->ia.write.in;