fuse: simplify logic in fuse_notify_store() and fuse_retrieve()

fuse: simplify logic in fuse_notify_store() and fuse_retrieve()

Simplify the folio parsing logic in fuse_notify_store() and
fuse_retrieve().

In particular, calculate the index by tracking pos, which allows us to
remove calculating nr_pages, and use "pos" in place of outarg's offset
field.

Suggested-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>