CHROMIUM: Fix memcg path in MGLRU reclaim

Original implementation of the memcg path was not initiated properly.
Added a check to set it to an empty string to prevent printing random
strings.

BUG=b:254153610
TEST=Manual testing on DUT with both memcg on/off
Signed-off-by: Bryan Yu <yuholong@google.com>

Change-Id: I65c9be7d7b07c8bbcbc8009259db5d4dcd8d9e99
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4004384


Reviewed-by: Suleiman Souhlal <suleiman@chromium.org>
Reviewed-by: Kameron Lutes <kalutes@chromium.org>
Auto-Submit: Bryan Yu <yuholong@google.com>
Commit-Queue: Bryan Yu <yuholong@google.com>
Reviewed-by: Alexandre Marciano Gimenez <raging@google.com>
Tested-by: Bryan Yu <yuholong@google.com>