lib: suppress error msg when filling the cache

lib: suppress error msg when filling the cache

Before the patch:
$ ip netns add foo
$ ip link add name veth1 address 2a:a5:5c:b9:52:89 type veth peer name veth2 address 2a:a5:5c:b9:53:90 netns foo
RTNETLINK answers: No such device
RTNETLINK answers: No such device

But the command was successful. This may break script. Let's remove those
error messages.

Fixes: 55870dfe7f8b ("Improve batch and dump times by caching link lookups")
Reported-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>