cifs: use a flexarray in cifs_writedata

The cifs_writedata code uses a single element trailing array, which
just adds unneeded complexity. Use a flexarray instead.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>