rtnetlink: Check for allocation failure in _nl_store_ifp_cookie()

[pouria]

Summary

Almost impossible to happen, but if it does, just report nothing.

MFC after: 3 days

Test Plan

Follow up D56954

[markj]

Hmm, if we think it's impossible, then probably it's better to just assume nla_cookie != NULL rather than silently doing nothing? Otherwise we should probably report the error back to create_link(), no?

[pouria]

In D56965#1305148, @markj wrote:

Hmm, if we think it's impossible, then probably it's better to just assume nla_cookie != NULL rather than silently doing nothing? Otherwise we should probably report the error back to create_link(), no?

It's absolutely safe to assume nla_cookie != NULL.
Unless someone else touches this code and forgets about the null check.
We can leave it as is.