fs: Add safe locking semantics for format.json (#4523)

This patch also reverts previous changes which were
merged for migration to the newer disk format. We will
be bringing these changes in subsequent releases. But
we wish to add protection in this release such that
future release migrations are protected.

Revert "fs: Migration should handle bucketConfigs as regular objects. (#4482)"
This reverts commit 976870a391.

Revert "fs: Migrate object metadata to objects directory. (#4195)"
This reverts commit 76f4f20609.

pkg/lock/lock.go

@@ -19,10 +19,16 @@
 package lock
 
 import (
+	"errors"
 	"os"
 	"sync"
 )
 
+var (
+	// ErrAlreadyLocked is returned if the underlying fd is already locked.
+	ErrAlreadyLocked = errors.New("file already locked")
+)
+
 // RLockedFile represents a read locked file, implements a special
 // closer which only closes the associated *os.File when the ref count.
 // has reached zero, i.e when all the readers have given up their locks.