UDP: Don't keep retrying send if we get EINVAL

UDP can return EINVAL if the route is down or not configured, this can
cause knet to 'spin' (well, ish, there is a usleep in there).

With this patch in place knet doesn't hog the CPU anywhere near so much
(and it does recover if the link returns).

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>