compat: Fixup ipv6 fragmentation on 4.9.135+ kernels

compat: Fixup ipv6 fragmentation on 4.9.135+ kernels

Upstream commit 648700f76b03 ("inet: frags: use rhashtables...") changed
how ipv6 fragmentation is implemented.  This patch was backported to
the upstream stable 4.9.x kernel starting at 4.9.135.

This patch creates the compatibility layer changes required to both
compile and also operate correctly with ipv6 fragmentation on these
kernels. Check if the inet_frags 'rnd' field is present to key on
whether the upstream patch is present.  Also update Travis to the
latest 4.9 kernel release so that this patch is compile tested.

Passes Travis:
https://travis-ci.org/gvrose8192/ovs-experimental/builds/478033409

Cc: William Tu <u9012063@gmail.com>
Cc: Yi-Hung Wei <yihung.wei@gmail.com>
Cc: Yifeng Sun <pkusunyifeng@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>