netfs: Fix writethrough to use collection offload

Fix writethrough write to set NETFS_RREQ_OFFLOAD_COLLECTION on the request
so that collection is processed asynchronously rather than only right at
the end - and also so that asynchronous O_SYNC writes get collected at all.

Fixes: 288ace2f57c9 ("netfs: New writeback implementation")
Closes: https://sashiko.dev/#/patchset/20260616100821.2062304-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org