netd bpf: fix an error discovered by the next compiler update.

flags is uint16_t, but only the low byte is initialized and used.
Before the next compiler update, clang optimizes flags to uint8_t,
and thus the kernel doesn't complain.

But in the next compiler update, clang no longer tries to optimize
flags to uint8_t, because of https://reviews.llvm.org/D73997.
Then the kernel fails to load netd.o. Because netd.o tries to read the
upper byte of flags which was never initialized.

Bug: 149839606
Test: build and load netd.o successfully.

Change-Id: Ife6e2d7d8c3738e33841989a7f1ee7401bc3c963
Merged-In: Ife6e2d7d8c3738e33841989a7f1ee7401bc3c963