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.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/config

@@ -8,6 +8,7 @@ CONFIG_BPF_LIRC_MODE2=y
 CONFIG_BPF_LSM=y
 CONFIG_BPF_STREAM_PARSER=y
 CONFIG_BPF_SYSCALL=y
+CONFIG_BPF_UNPRIV_DEFAULT_OFF=n
 CONFIG_CGROUP_BPF=y
 CONFIG_CRYPTO_HMAC=y
 CONFIG_CRYPTO_SHA256=y