fuse: fix missing unlock_page in fuse_writepage()

BugLink: https://bugs.launchpad.net/bugs/1848589

commit d5880c7a upstream.

unlock_page() was missing in case of an already in-flight write against the
same page.
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Fixes: ff17be08 ("fuse: writepage: skip already in flight")
Cc: <stable@vger.kernel.org> # v3.13
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/fuse/file.c

@@ -1706,6 +1706,7 @@ static int fuse_writepage(struct page *page, struct writeback_control *wbc)
 		WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
 
 		redirty_page_for_writepage(wbc, page);
+		unlock_page(page);
 		return 0;
 	}