IPv4: RTM_GETROUTE: Add RTA_ENCAP to result

This patch adds an IPv4 routes encapsulation attribute
to the result of netlink RTM_GETROUTE requests
(e.g. ip route get 192.0.2.1).

Signed-off-by: Oliver Herms <oliver.peter.herms@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20201113085517.GA1307262@tws


Signed-off-by: Jakub Kicinski <kuba@kernel.org>