platform: don't consult cache before invoking netlink operation
Checking whether the link exists in the cache, before talking to kernel
serves no purpose.
- in all cases, the caller already has a good indication that the link
in fact exists. That is, because the caller makes decisions on what to
do, based on what platform told it earlier. Thus, the check usually succeeds
anyway.
- in the unexpected case it doesn't succeed, we
- should not silently return without logging at least a message
- we possibly still want to send the netlink message to kernel,
just to have it fail. Note that the ifindex is indeed the identifier
for the link, so there is no danger of accidentally killing the
wrong link.
Well, theoretically there is, because the kernel's ifindex counter can
wrap or get reused when moving links between namespaces. But checking
the cache would not protect against that anyway! Worst case, the cache
would already have the impostor link and would not prevent from doing
the wrong thing. After all, they do have the same identifier, so how
would we know that this is in fact a different link?