xfrm: Fix local error reporting crash with interfamily tunnels

We set the outer mode protocol too early. As a result, the
local error handler might dispatch to the wrong address family
and report the error to a wrong socket type. We fix this by
setting the outer protocol to the skb after we accessed the
inner mode for the last time, right before we do the atcual
encapsulation where we switch finally to the outer mode.

Reported-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Tested-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>