speedup getFormatErasureInQuorum use driveCount (#14239)

startup speed-up, currently getFormatErasureInQuorum()
would spend up to 2-3secs when there are 3000+ drives
for example in a setup, simplify this implementation
to use drive counts.

cmd/xl-storage.go

@@ -216,14 +216,10 @@ func newXLStorage(ep Endpoint) (s *xlStorage, err error) {
 	if globalIsCICD {
 		rootDisk = true
 	} else {
-		rootDisk, err = disk.IsRootDisk(path, SlashSeparator)
-		if err != nil {
-			return nil, err
-		}
-		if !rootDisk && globalRootDiskThreshold > 0 {
-			// If for some reason we couldn't detect the root disk
-			// use - MINIO_ROOTDISK_THRESHOLD_SIZE to figure out if
-			// this disk is a root disk.
+		if globalRootDiskThreshold > 0 {
+			// When you do not want rely on automatic verification
+			// of rejecting root disks, we need to add this threshold
+			// to ensure that root disks are ignored properly.
 			info, err := disk.GetInfo(path)
 			if err != nil {
 				return nil, err
@@ -232,6 +228,14 @@ func newXLStorage(ep Endpoint) (s *xlStorage, err error) {
 			// treat those disks with size less than or equal to the
 			// threshold as rootDisks.
 			rootDisk = info.Total <= globalRootDiskThreshold
+		} else {
+			// When root disk threshold is not set, we rely
+			// on automatic detection - does not work in
+			// container environments.
+			rootDisk, err = disk.IsRootDisk(path, SlashSeparator)
+			if err != nil {
+				return nil, err
+			}
 		}
 	}