xfrm: Fix pmtu discovery for local generated packets.

Commit 044a832a ("xfrm: Fix local error reporting crash
with interfamily tunnels") moved the setting of skb->protocol
behind the last access of the inner mode family to fix an
interfamily crash. Unfortunately now skb->protocol might not
be set at all, so we fail dispatch to the inner address family.
As a reault, the local error handler is not called and the
mtu value is not reported back to userspace.

We fix this by setting skb->protocol on message size errors
before we call xfrm_local_error.

Fixes: 044a832a

 ("xfrm: Fix local error reporting crash with interfamily tunnels")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>