selftests/bpf: Use "is not set" instead of "=n"

"=n" is not valid kconfig syntax. Use "is not set" instead to indicate
the option should be disabled.
Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221205131618.1524337-3-daan.j.demeyer@gmail.com

tools/testing/selftests/bpf/config

@@ -8,7 +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_BPF_UNPRIV_DEFAULT_OFF is not set
 CONFIG_CGROUP_BPF=y
 CONFIG_CRYPTO_HMAC=y
 CONFIG_CRYPTO_SHA256=y