bridge: netfilter: fix update_pmtu crash with GRE

bridge: netfilter: fix update_pmtu crash with GRE

[ Upstream commit 631339f1e544a4d39a63cfe6708c5bddcd5a2c48 ]

As GRE tries to call the update_pmtu function on skb->dst and
bridge supplies an skb->dst that has a NULL ops field, all is
not well.

This patch fixes this by giving the bridge device an ops field
with an update_pmtu function.  For the moment I've left all
other fields blank but we can fill them in later should the
need arise.

Based on report and patch by Philip Craig.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>