add root_disk threshold detection (#12259)

as there is no automatic way to detect if there
is a root disk mounted on / or /var for the container
environments due to how the root disk information
is masked inside overlay root inside container.

this PR brings an environment variable to set
root disk size threshold manually to detect the
root disks in such situations.

.github/workflows/go.yml

@@ -34,6 +34,11 @@ jobs:
         env:
           CGO_ENABLED: 0
           GO111MODULE: on
+          MINIO_KMS_KES_CERT_FILE: /home/runner/work/minio/minio/.github/workflows/root.cert
+          MINIO_KMS_KES_KEY_FILE: /home/runner/work/minio/minio/.github/workflows/root.key
+          MINIO_KMS_KES_ENDPOINT: "https://play.min.io:7373"
+          MINIO_KMS_KES_KEY_NAME: "my-minio-key"
+          MINIO_KMS_AUTO_ENCRYPTION: on
         run: |
           sudo sysctl net.ipv6.conf.all.disable_ipv6=0
           sudo sysctl net.ipv6.conf.default.disable_ipv6=0