fs: move kmem_cache_zalloc() into alloc_empty_file*() helpers

Use a common helper init_file() instead of __alloc_file() for
alloc_empty_file*() helpers and improrve the documentation.

This is needed for a follow up patch that allocates a backing_file
container.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230615112229.2143178-4-amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>