de-couple walkMu and walkReadMu for some granularity (#13231)

This commit brings two locks instead of single lock for
WalkDir() calls on top of c25816eabc.

The main reason is to avoid contention between readMetadata()
and ListDir() calls, ListDir() can take time on prefixes that
are huge for readdir() but this shouldn't end up blocking
all readMetadata() operations, this allows for more room for
I/O while not overly penalizing all listing operations.

cmd/xl-storage.go

@@ -142,7 +142,8 @@ type xlStorage struct {
 	sync.RWMutex
 
 	// mutex to prevent concurrent read operations overloading walks.
-	walkMu sync.Mutex
+	walkMu     sync.Mutex
+	walkReadMu sync.Mutex
 }
 
 // checkPathLength - returns error if given path name length more than 255