Don't decrypt ETag in validation when source is SSEC multipart (#7423)

Copying an encrypted SSEC object when this latter is uploaded using
multipart mechanism was failing because ETag in case of encrypted
multipart upload is not encrypted.

This PR fixes the behavior.

cmd/object-handlers-common.go

@@ -98,13 +98,14 @@ func checkCopyObjectPreconditions(ctx context.Context, w http.ResponseWriter, r
 		}
 	}
 
-	ssec := crypto.SSECopy.IsRequested(r.Header)
+	shouldDecryptEtag := crypto.SSECopy.IsRequested(r.Header) && !crypto.IsMultiPart(objInfo.UserDefined)
+
 	// x-amz-copy-source-if-match : Return the object only if its entity tag (ETag) is the
 	// same as the one specified; otherwise return a 412 (precondition failed).
 	ifMatchETagHeader := r.Header.Get("x-amz-copy-source-if-match")
 	if ifMatchETagHeader != "" {
 		etag := objInfo.ETag
-		if ssec {
+		if shouldDecryptEtag {
 			etag = encETag[len(encETag)-32:]
 		}
 		if objInfo.ETag != "" && !isETagEqual(etag, ifMatchETagHeader) {
@@ -120,7 +121,7 @@ func checkCopyObjectPreconditions(ctx context.Context, w http.ResponseWriter, r
 	ifNoneMatchETagHeader := r.Header.Get("x-amz-copy-source-if-none-match")
 	if ifNoneMatchETagHeader != "" {
 		etag := objInfo.ETag
-		if ssec {
+		if shouldDecryptEtag {
 			etag = encETag[len(encETag)-32:]
 		}
 		if objInfo.ETag != "" && isETagEqual(etag, ifNoneMatchETagHeader) {