openvswitch: Fix IPv6 later frags parsing

The previous commit fa642f08


("openvswitch: Derive IP protocol number for IPv6 later frags")
introduces IP protocol number parsing for IPv6 later frags that can mess
up the network header length calculation logic, i.e. nh_len < 0.
However, the network header length calculation is mainly for deriving
the transport layer header in the key extraction process which the later
fragment does not apply.

Therefore, this commit skips the network header length calculation to
fix the issue.

Reported-by: Chris Mi <chrism@mellanox.com>
Reported-by: Greg Rose <gvrose8192@gmail.com>
Fixes: fa642f08

 ("openvswitch: Derive IP protocol number for IPv6 later frags")
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>