staging: erofs: set sb->s_root to NULL when failing from __getname()

Set sb->s_root to NULL when failing from __getname(),
so that we can avoid double dput and unnecessary operations
in generic_shutdown_super().

Signed-off-by: Chengguang Xu <cgxu519@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>