files: use PROC_SELF when a process accesses its /proc/PID

If a process is executed in another pidns, a /proc/PID doesn't link with
the proper process.

This patch fixes a problem like this:
   1: Error (util.c:106): Unable to close fd 33: Bad file descriptor
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files.c

@@ -938,7 +938,7 @@ int close_old_fds(struct pstree_item *me)
 	struct dirent *de;
 	int fd, ret;
 
-	dir = opendir_proc(getpid(), "fd");
+	dir = opendir_proc(PROC_SELF, "fd");
 	if (dir == NULL)
 		return -1;