fuse: Replace page without copying in fuse_writepage_in_flight()

It looks like we can optimize page replacement and avoid copying by simple
updating the request's page.

[SzM: swap with new request's tmp page to avoid use after free.]
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1789,7 +1789,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
 		if (curr_index == page->index) {
 			WARN_ON(tmp->num_pages != 1);
 			WARN_ON(!test_bit(FR_PENDING, &tmp->flags));
-			copy_highpage(tmp->pages[0], page);
+			swap(tmp->pages[0], new_req->pages[0]);
 			break;
 		}
 	}