BpfMap - fix a clang warning: misc-unconventional-assign-operator am: 8ed7c0f9b9

Original change: https://android-review.googlesource.com/c/platform/system/bpf/+/1552943

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I4c8a9e299d9f2d8399c03156caffb595037ab443