ESP: Export esp_output_fill_trailer function

The esp fill trailer method is identical for both
IPv6 and IPv4.

Share the implementation for esp6 and esp to avoid
code duplication in addition it could be also used
at various drivers code.

Signed-off-by: Raed Salem <raeds@mellanox.com>
Reviewed-by: Boris Pismenny <borisp@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>