s3:smb2: add padding to last command in compound requests

s3:smb2: add padding to last command in compound requests

Following Windows behaviour, the last command in a compound request
should be padded to an 8 byte boundary and OS X clients crash badly if
we don't pad.

[MS-SMB2] 3.3.4.1.3, "Sending Compounded Responses", doesn't make it
clear whether the padding requirement governs the last command in a
compound response, a future MS-SMB2 update will document Windwows
product behaviour in a footnote.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11277

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit dfa64b958b201931e0dbe11f153f606f20217594)