openvswitch: Avoid OOB read when parsing flow nlattrs

[ Upstream commit 04a4af334b971814eedf4e4a413343ad3287d9a9 ]

For nested and variable attributes, the expected length of an attribute
is not known and marked by a negative number.  This results in an OOB
read when the expected length is later used to check if the attribute is
all zeros. Fix this by using the actual length of the attribute rather
than the expected length.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>