bpf: disallow bpf tc programs access current->pid,uid

Accessing current->pid/uid from cls_bpf may lead to misleading results and
should not be used when TC classifiers need accurate information about pid/uid.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>