mm/damon: don't use strnlen() with known-bogus source length

mainline inclusion
from mainline-5.15-rc3
commit 892ab4bb
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4GVMK
CVE: NA

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gcc knows the true length too, and rightfully complains.

Link: https://lkml.kernel.org/r/20210912204447.10427-1-kilobyte@angband.pl


Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Cc: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 892ab4bb)
Signed-off-by: Yue Zou <zouyue3@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>