CHROMIUM: Bug fix for implementation of MGLRU in sysfs

Fix the typo in CL:3965160 where it should be buf_len instead of buf.

BUG=b:254153610
TEST=Manual testing on DUT

Signed-off-by: Bryan Yu <yuholong@google.com>
Change-Id: I5564b63cc19decd5eff69be9910271e0c95661e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3995292


Reviewed-by: Kameron Lutes <kalutes@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Alexandre Marciano Gimenez <raging@google.com>