fs: add helper to use mount option as path or fd

Allow filesystems to use a mount option either as a
file or path.

Link: https://lore.kernel.org/r/20241014-work-overlayfs-v3-1-32b3fed1286e@kernel.org


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>