fallback on etags if they match when mtime is not same (#17424)

on "unversioned" buckets there are situations
when successive concurrent I/O can lead to
an inconsistent state() with mtime while the
etag might be the same for the object on disk.

in such a scenario it is possible for us to
allow reading of the object since etag matches
and if etag matches we are guaranteed that we
have enough copies the object will be readable
and same.

This PR allows fallback in such scenarios.

cmd/erasure-metadata_test.go

@@ -204,7 +204,7 @@ func TestFindFileInfoInQuorum(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run("", func(t *testing.T) {
-			_, err := findFileInfoInQuorum(context.Background(), test.fis, test.modTime, test.expectedQuorum)
+			_, err := findFileInfoInQuorum(context.Background(), test.fis, test.modTime, "", test.expectedQuorum)
 			if err != test.expectedErr {
 				t.Errorf("Expected %s, got %s", test.expectedErr, err)
 			}