lib/tc: Support optional tunnel id

lib/tc: Support optional tunnel id

Currently the TC tunnel_key action is always
initialized with the given tunnel id value. However,
some tunneling protocols define the tunnel id as an optional field.

This patch initializes the id field of tunnel_key:set and tunnel_key:unset
only if a value is provided.

In the case that a tunnel key value is not provided by the user
the key flag will not be set.

Signed-off-by: Adi Nissim <adin@mellanox.com>
Acked-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>