ping6: Fix send to link-local addresses with VRF.

ping6: Fix send to link-local addresses with VRF.

Ping sockets can't send packets when they're bound to a VRF master
device and the output interface is set to a slave device.

For example, when net.ipv4.ping_group_range is properly set, so that
ping6 can use ping sockets, the following kind of commands fails:
  $ ip vrf exec red ping6 fe80::854:e7ff:fe88:4bf1%eth1

What happens is that sk->sk_bound_dev_if is set to the VRF master
device, but 'oif' is set to the real output device. Since both are set
but different, ping_v6_sendmsg() sees their value as inconsistent and
fails.

Fix this by allowing 'oif' to be a slave device of ->sk_bound_dev_if.

This fixes the following kselftest failure:
  $ ./fcnal-test.sh -t ipv6_ping
  [...]
  TEST: ping out, vrf device+address bind - ns-B IPv6 LLA        [FAIL]

Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/b6191f90-ffca-dbca-7d06-88a9788def9c@alu.unizg.hr/
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Fixes: 5e457896986e ("net: ipv6: Fix ping to link-local addresses.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/6c8b53108816a8d0d5705ae37bdc5a8322b5e3d9.1686153846.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>