fuse: Add missed unlock_page() to fuse_readpages_fill()

BugLink: https://bugs.launchpad.net/bugs/1792392

commit 109728cc upstream.

The above error path returns with page unlocked, so this place seems also
to behave the same.

Fixes: f8dbdf81 ("fuse: rework fuse_readpages()")
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/fuse/file.c

@@ -879,6 +879,7 @@ static int fuse_readpages_fill(void *_data, struct page *page)
 	}
 
 	if (WARN_ON(req->num_pages >= req->max_pages)) {
+		unlock_page(page);
 		fuse_put_request(fc, req);
 		return -EIO;
 	}