selftests/bpf: check return value of fopen in test_verifier.c

Commit 0a674874 ("selftests/bpf: Only run tests if !bpf_disabled")
forgot to check return value of fopen.

This caused some confusion, when running test_verifier (from
tools/testing/selftests/bpf/) on an older kernel (< v4.4) as it will
simply seqfault.

This fix avoids the segfault and prints an error, but allow program to
continue.  Given the sysctl was introduced in 1be7f75d ("bpf:
enable non-root eBPF programs"), we know that the running kernel
cannot support unpriv, thus continue with unpriv_disabled = true.

Fixes: 0a674874 ("selftests/bpf: Only run tests if !bpf_disabled")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>