iomap: add a private argument for iomap_file_buffered_write

In order to switch fuse over to using iomap for buffered writes we need
to be able to have the struct file for the original write, in case we
have to read in the page to make it uptodate.  Handle this by using the
existing private field in the iomap_iter, and add the argument to
iomap_file_buffered_write.  This will allow us to pass the file in
through the iomap buffered write path, and is flexible for any other
file systems needs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/7f55c7c32275004ba00cddf862d970e6e633f750.1724755651.git.josef@toxicpanda.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>