Do not use O_EXCL for allocated backing loop device.

Exclusive flag is defined only when creating a file,
for opening existing file it is undefinded operation.

Remove it from crypt_loop_attach as it was wrong since
the initial commit.

lib/utils_loop.c

@@ -112,10 +112,10 @@ int crypt_loop_attach(char **loop, const char *file, int offset,
 
 	*loop = NULL;
 
-	file_fd = open(file, (*readonly ? O_RDONLY : O_RDWR) | O_EXCL);
+	file_fd = open(file, *readonly ? O_RDONLY : O_RDWR);
 	if (file_fd < 0 && (errno == EROFS || errno == EACCES) && !*readonly) {
 		*readonly = 1;
-		file_fd = open(file, O_RDONLY | O_EXCL);
+		file_fd = open(file, O_RDONLY);
 	}
 	if (file_fd < 0)
 		goto out;