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

In order to support compounding and encryption we need to separate
RFC1001 length field and SMB2 header structure because the protocol
treats them differently. This change will allow to simplify parsing
of such complex SMB2 packets further.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
(cherry picked from commit 31473fc4)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>