ip/tunnel: introduce tnl_parse_key()

ip/tunnel: introduce tnl_parse_key()

Instead of duplicating the same code six times (key, ikey and okey in
iptunnel and ip6tunnel), have a common parsing routine. This has the
added benefit of having the same verbose error message in ip6tunnel as
well as iptunnel.

I'm not sure if parsing an IPv4 address as key makes sense for
ip6tunnel, but the code was there before so this patch at least doesn't
make it worse.

Signed-off-by: Phil Sutter <phil@nwl.cc>