SMB3.1.1: remove confusing mount warning when no SPNEGO info on negprot rsp

SMB3.1.1: remove confusing mount warning when no SPNEGO info on negprot rsp

BugLink: https://bugs.launchpad.net/bugs/1910111
commit bc7c4129d4cdc56d1b5477c1714246f27df914dd upstream.

Azure does not send an SPNEGO blob in the negotiate protocol response,
so we shouldn't assume that it is there when validating the location
of the first negotiate context.  This avoids the potential confusing
mount warning:

   CIFS: Invalid negotiate context offset

CC: Stable <stable@vger.kernel.org>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>