smb: client: set missing retry flag in cifs_writev_callback()

Set NETFS_SREQ_NEED_RETRY flag to tell netfslib that the subreq needs
to be retried.

Fixes: ee4cdf7b ("netfs: Speed up buffered reading")
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250701163852.2171681-9-dhowells@redhat.com


Tested-by: Steve French <sfrench@samba.org>
Cc: linux-cifs@vger.kernel.org
Cc: netfs@lists.linux.dev
Signed-off-by: Christian Brauner <brauner@kernel.org>