show a notice when mixed rootfs & mounted disks is detected (#9471)

A user can incorrectly mounts a newly fresh disk. MinIO will detect
that it is writing with a rootfs disk and will mark it down. However,
it is hard for the user to understand what's going on.

This commit will just print a notice so it will be easy to spot
such use case.

cmd/xl-sets.go

@@ -1459,6 +1459,8 @@ func markRootDisksAsDown(storageDisks []StorageAPI) {
 		if infos[i].RootDisk {
 			// We should not heal on root disk. i.e in a situation where the minio-administrator has unmounted a
 			// defective drive we should not heal a path on the root disk.
+			logger.Info("Disk `%s` is a root disk. Please ensure the disk is mounted properly, refusing to use root disk.",
+				storageDisks[i].String())
 			storageDisks[i] = nil
 		}
 	}