staging: erofs: remove rcu_read_lock() in erofs_try_to_free_cached_page

page_private(page) cannot be changed if page lock is taken.

Besides, the corresponding workgroup won't be freed
if the page is already protected by page lock, therefore
no need to take rcu read lock.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>