writeback: remove redundant checks for root memcg

The check for root memcg will be done in wb_get_lookup(), so remove the
redundant one to simplify the code.

Link: https://lkml.kernel.org/r/20230808084431.1632934-1-alexjlzheng@tencent.com


Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>