mm/workingset.c: fix some doc warnings

mm/workingset.c: fix some doc warnings

Fix following warnings caused by mismatch bewteen function parameters and
comments.

mm/workingset.c:228: warning: Function parameter or member 'lruvec' not described in 'workingset_age_nonresident'
mm/workingset.c:228: warning: Excess function parameter 'memcg' description in 'workingset_age_nonresident'

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/1600485913-11192-1-git-send-email-tanxiaofei@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>