CIFS: Set ->should_dirty in cifs_user_readv()

The current code causes a static checker warning because ITER_IOVEC is
zero so the condition is never true.

Fixes: 6685c5e2 ("CIFS: Add asynchronous read support through kernel AIO")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>