fuse: fix premature writetrhough request for large folio

fuse: fix premature writetrhough request for large folio

When large folio is enabled and the initial folio offset exceeds
PAGE_SIZE, e.g. the position resides in the second page of a large
folio, after the folio copying the offset (in the page) won't be updated
to 0 even though the expected range is successfully copied until the end
of the folio.  In this case fuse_fill_write_pages() exits prematurelly
before the request has reached the max_write/max_pages limit.

Fix this by eliminating page offset entirely and use folio offset
instead.

Fixes: d60a6015e1a2 ("fuse: support large folios for writethrough writes")
Reviewed-by: Horst Birthelmer <hbirthelmer@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>