netfs_read_folio() needs to handle partially-valid pages that are marked
dirty, but not uptodate in the event that someone tries to read a page was
used to cache data by a streaming write.

In such a case, make netfs_read_folio() set up a bvec iterator that points
to the parts of the folio that need filling and to a sink page for the data
that should be discarded and use that instead of i_pages as the iterator to
be written to.

This requires netfs_rreq_unlock_folios() to convert the page into a normal
dirty uptodate page, getting rid of the partial write record and bumping
the group pointer over to folio->private.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org