lib/libnetlink: update rtnl_talk to support malloc buff at run time

lib/libnetlink: update rtnl_talk to support malloc buff at run time

This is an update for 460c03f3f3cc ("iplink: double the buffer size also in
iplink_get()"). After update, we will not need to double the buffer size
every time when VFs number increased.

With call like rtnl_talk(&rth, &req.n, NULL, 0), we can simply remove the
length parameter.

With call like rtnl_talk(&rth, nlh, nlh, sizeof(req), I add a new variable
answer to avoid overwrite data in nlh, because it may has more info after
nlh. also this will avoid nlh buffer not enough issue.

We need to free answer after using.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>