cifs: fix protocol definition for READ_RSP

There is no pad, and it simplifies the code to remove the "Data" field.

None of the existing code relies on these fields, or on the READ_RSP
being a particular length.

Reviewed-and-Tested-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Jeff Layton <jlayton@redhat.com>