A writeable mapped backing file can perform writes to the real inode.
Therefore, the real path mount must be kept writable so long as the
writable map exists.

This may not be strictly needed for ovelrayfs private upper mount,
but it is correct to take the mnt_writers count in the vfs helper.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231009153712.1566422-2-amir73il@gmail.comSigned-off-by: Christian Brauner <brauner@kernel.org>