ntfs3: use file_mnt_idmap helper

ntfs3: 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: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Cc: <ntfs3@lists.linux.dev>
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/20240129180024.219766-1-aleksandr.mikhalitsyn@canonical.com
Signed-off-by: Christian Brauner <brauner@kernel.org>