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/api-errors.go

@@ -1956,6 +1956,8 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 		apiErr = ErrNoSuchKey
 	case MethodNotAllowed:
 		apiErr = ErrMethodNotAllowed
+	case ObjectLocked:
+		apiErr = ErrObjectLocked
 	case InvalidVersionID:
 		apiErr = ErrInvalidVersionID
 	case VersionNotFound: