bpf: allow socket_filter programs to use bpf_prog_test_run

in order to improve test coverage allow socket_filter program type
to be run via bpf_prog_test_run command.
Since such programs can be loaded by non-root tighten
permissions for bpf_prog_test_run to be root only
to avoid surprises.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>