remove double reads updating object metadata (#13542)

Removes RLock/RUnlock for updating metadata,
since we already take a write lock to update
metadata, this change removes reading of xl.meta
as well as an additional lock, the performance gain
should increase 3x theoretically for

- PutObjectRetention
- PutObjectLegalHold

This optimization is mainly for Veeam like
workloads that require a certain level of iops
from these API calls, we were losing iops.

cmd/object-api-errors.go

@@ -291,6 +291,13 @@ func (e MethodNotAllowed) Error() string {
 	return "Method not allowed: " + e.Bucket + "/" + e.Object
 }
 
+// ObjectLocked object is currently WORM protected.
+type ObjectLocked GenericError
+
+func (e ObjectLocked) Error() string {
+	return "Object is WORM protected and cannot be overwritten: " + e.Bucket + "/" + e.Object + "(" + e.VersionID + ")"
+}
+
 // ObjectAlreadyExists object already exists.
 type ObjectAlreadyExists GenericError