gre/gre6: Unify local/remote endpoint address parsing

gre/gre6: Unify local/remote endpoint address parsing

We are going to merge link_gre.c and link_gre6.c and this is final step
to make their diffs clear and show what needs to be changed during merge.

Note that it is safe to omit endpoint address(es) from netlink create
request as kernel is aware of such case and will use zero for that
endpoint(s).

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>