[CIFS] Fix set file size to zero when doing chmod to Samba 3.0.26pre

In fixing a bug Samba 3.0.26pre allowed some clients (including Linux cifs
client) to change file size to zero in SET_FILE_UNIX_BASIC (which Linux cifs
client uses for chmod).

The server has been "fixed" now but that also fixes the client to net send
file size zero on chmod.

Fixes Samba bugzilla bug # 4418.

Fixed with help from Jeremy Allison

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>