selftests/bpf: add checks on extack messages for eBPF hw offload tests

Add checks to test that netlink extack messages are correctly displayed
in some expected error cases for eBPF offload to netdevsim with TC and
XDP.

iproute2 may be built without libmnl support, in which case the extack
messages will not be reported.  Try to detect this condition, and when
enountered print a mild warning to the user and skip the extack validation.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>