datapath: fix missing checks for nla_nest_start

datapath: fix missing checks for nla_nest_start

Upstream commit:
    commit 0fff9bd47e1341b5c4db862cc39fc68ce45f165d
    Author: Kangjie Lu <kjlu@umn.edu>
    Date:   Fri Mar 15 01:11:22 2019 -0500

    net: openvswitch: fix missing checks for nla_nest_start

    nla_nest_start may fail and thus deserves a check.
    The fix returns -EMSGSIZE when it fails.

    Signed-off-by: Kangjie Lu <kjlu@umn.edu>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>