net/sched: act_tunnel_key: disambiguate metadata dst error cases

Metadata may be NULL for one of two reasons:
* Missing user input
* Failure to allocate the metadata dst

Disambiguate these case by returning -EINVAL for the former and -ENOMEM
for the latter rather than -EINVAL for both cases.

This is in preparation for using extended ack to provide more information
to users when parsing their input.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>