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>