fuse: support large folios for stores

Add support for folios larger than one page size for stores.
Also change variable naming from "this_num" to "nr_bytes".

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>