fuse: fix direct io folio offset and length calculation

For the direct io case, the pages from userspace may be part of a huge
folio, even if all folios in the page cache for fuse are small.

Fix the logic for calculating the offset and length of the folio for
the direct io case, which currently incorrectly assumes that all folios
encountered are one page size.

Fixes: 3b97c365 ("fuse: convert direct io to use folios")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>