io_uring: use file_mnt_idmap helper

Let's use file_mnt_idmap() as we do that across the tree.

No functional impact.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: io-uring@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Link: https://lore.kernel.org/r/20240129180024.219766-2-aleksandr.mikhalitsyn@canonical.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/xattr.c

@@ -112,7 +112,7 @@ int io_fgetxattr(struct io_kiocb *req, unsigned int issue_flags)
 
 	WARN_ON_ONCE(issue_flags & IO_URING_F_NONBLOCK);
 
-	ret = do_getxattr(mnt_idmap(req->file->f_path.mnt),
+	ret = do_getxattr(file_mnt_idmap(req->file),
 			req->file->f_path.dentry,
 			&ix->ctx);