staging: erofs: switch to ->iterate_shared()

After commit 61922694 ("introduce a parallel variant of ->iterate()"),
readdir can be done without taking exclusive inode lock of course.

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