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

In order to simplify further encryption support we need to separate
RFC1001 length and SMB2 header when sending a request. Put the length
field in iov[0] and the rest of the packet into following iovs.
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
(cherry picked from commit 738f9de5)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>