For GRE interfaces in 'external' mode, the kernel ignores all manual
settings like remote IP address or TTL. However, for some of those
attributes, kernel checks their value and does not allow them to be zero
(even though they're ignored later).

Currently, 'ip link' always includes all attributes in the netlink message.
This leads to problem with creating interfaces in 'external' mode. For
example, this command does not work:

ip link add gre1 type gretap external

and needs a bogus remote IP address to be specified, as the kernel enforces
remote IP address to be either not present, or not null.

Ignore the parameters that do not make sense in 'external' mode.
Unfortunately, we cannot error out, as there may be existing deployments
that workarounded the bug by specifying bogus values.

Fixes: 926b39e1 ("gre: add support for collect metadata flag")
Signed-off-by: Jiri Benc <jbenc@redhat.com>