vrf: Fix use after free and double free in vrf_finish_output

commit 82dd0d2a9a76fc8fa2b18d80b987d455728bf83a upstream.

Miguel reported an skb use after free / double free in vrf_finish_output
when neigh_output returns an error. The vrf driver should return after
the call to neigh_output as it takes over the skb on error path as well.

Patch is a simplified version of Miguel's patch which was written for 4.9,
and updated to top of tree.

Fixes: 8f58336d ("net: Add ethernet header for pass through VRF device")
Signed-off-by: Miguel Fadon Perlines <mfadon@teldat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ backport to 4.4 and 4.9 dropped the sock_confirm_neigh and
  changed neigh_output to dst_neigh_output ]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>