CIFS: Separate SMB2 header structure

CIFS: Separate SMB2 header structure

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 31473fc4f9653b73750d3792ffce6a6e1bdf0da7)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>