net: ipv6: Fix ping to link-local addresses.

ping_v6_sendmsg does not set flowi6_oif in response to
sin6_scope_id or sk_bound_dev_if, so it is not possible to use
these APIs to ping an IPv6 address on a different interface.
Instead, it sets flowi6_iif, which is incorrect but harmless.

Stop setting flowi6_iif, and support various ways of setting oif
in the same priority order used by udpv6_sendmsg.

[Backport of net 5e457896986e16c440c97bb94b9ccd95dd157292]

Bug: 29370996
Change-Id: I2c8bc213c417a4427f64439e0954138cb30416c2
Tested: https://android-review.googlesource.com/#/c/254470/


Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>