fs: 9p: fix v9fs_file_open writeback fid error check

IS_ERR() and PTR_ERR() use wrong pointer, it should be
writeback_fid, fix it.

Link: http://lkml.kernel.org/r/20210330130632.1054357-1-yangyingliang@huawei.com
Fixes: 5bfe97d7 ("9p: Fix writeback fid incorrectly being attached to dentry")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
[Dominique: adjusted commit summary]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/vfs_file.c

@@ -86,8 +86,8 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 		 * to work.
 		 */
 		writeback_fid = v9fs_writeback_fid(file_dentry(file));
-		if (IS_ERR(fid)) {
-			err = PTR_ERR(fid);
+		if (IS_ERR(writeback_fid)) {
+			err = PTR_ERR(writeback_fid);
 			mutex_unlock(&v9inode->v_mutex);
 			goto out_error;
 		}