selftests/bpf: allow unpriv bpf for selftests by default

selftests/bpf: allow unpriv bpf for selftests by default

Enable unprivileged bpf for selftests kernel by default.
This forces CI to run test_verifier tests in both privileged
and unprivileged modes.

The test_verifier.c:do_test uses sysctl kernel.unprivileged_bpf_disabled
to decide whether to run or to skip test cases in unprivileged mode.
The CONFIG_BPF_UNPRIV_DEFAULT_OFF controls the default value of the
kernel.unprivileged_bpf_disabled.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20221116015456.2461135-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>