seg6: factor out End lookup nexthop processing to a dedicated function

The End nexthop lookup/input operations are moved into a new helper
function named input_action_end_finish(). This avoids duplicating the
code needed to compute the nexthop in the different flavors of the End
behavior.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>