netfs: Fix io_uring based write-through

[This was included in v2 of 9b038d00, but
v1 got pushed instead]

Fix netfs_unbuffered_write_iter_locked() to set the total request length in
the netfs_io_request struct rather than leaving it as zero.

Fixes: 288ace2f

 ("netfs: New writeback implementation")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <stfrench@microsoft.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: Christian Brauner <brauner@kernel.org>
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240620173137.610345-2-dhowells@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>