net: ping6: Fix ping -6 with interface name

When passing interface parameter to ping -6:
$ ping -6 ::11:141:84:9 -I eth2
Results in:
PING ::11:141:84:10(::11:141:84:10) from ::11:141:84:9 eth2: 56 data bytes
ping: sendmsg: Invalid argument
ping: sendmsg: Invalid argument

Initialize the fl6's outgoing interface (OIF) before triggering
ip6_datagram_send_ctl. Don't wipe fl6 after ip6_datagram_send_ctl() as
changes in fl6 that may happen in the function are overwritten explicitly.
Update comment accordingly.

Fixes: 13651224

 ("net: ping6: support setting basic SOL_IPV6 options via cmsg")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220531084544.15126-1-tariqt@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>