selftests/bpf: fix unpriv_disabled check in test_verifier

selftests/bpf: fix unpriv_disabled check in test_verifier

Commit 1d56ade032a49 changed the function get_unpriv_disabled() to
return its results as a bool instead of updating a global variable, but
test_verifier was not updated to keep in line with these changes. Thus
unpriv_disabled is always false in test_verifier and unprivileged tests
are not properly skipped on systems with unprivileged bpf disabled.

Fixes: 1d56ade032a49 ("selftests/bpf: Unprivileged tests for test_loader.c")
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230912120631.213139-1-asavkov@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>