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

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>