BugLink: http://bugs.launchpad.net/bugs/1670508

Do not process RFC1001 length in smb2_hdr_assemble() because
it is not a part of SMB2 header. This allows to cleanup the code
and adds a possibility combine several SMB2 packets into one
for compounding.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
(cherry picked from commit cb200bd6)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>