eventpoll: use-after-possible-free in epoll_create1()

As soon as we'd installed the file into descriptor table, it can
get closed by another thread.  Freeing ep in process...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/eventpoll.c

@@ -1654,8 +1654,8 @@ SYSCALL_DEFINE1(epoll_create1, int, flags)
 		error = PTR_ERR(file);
 		goto out_free_fd;
 	}
-	fd_install(fd, file);
 	ep->file = file;
+	fd_install(fd, file);
 	return fd;
 
 out_free_fd: