fs: Export alloc_empty_backing_file

fs: Export alloc_empty_backing_file

There is no need to open nonexistent real files if backing files
couldn't be backed by real files (e.g., EROFS page cache sharing
doesn't need typical real files to open again).

Therefore, we export the alloc_empty_backing_file() helper, allowing
filesystems to dynamically set the backing file without real file
open. This is particularly useful for obtaining the correct @path
and @inode when calling file_user_path() and file_user_inode().

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>