smbd: Add padding byte to readx response

smbd: Add padding byte to readx response

MS-CIFS 2.2.4.42.2 states: "Pad (1 byte): This field is optional. When
using the NT LAN Manager dialect, this field can be used to align the
Data field to a 16-bit boundary relative to the start of the SMB Header.
If Unicode strings are being used, this field MUST be present. When
used, this field MUST be one padding byte long."

Always add the padding byte to all readx responses to avoid additional
complexity.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>