Fix the ignoring incomplete IPv6 neighbors logic.

We should only ignore the NUD failure from the incomplete IPv6
neighbors iff the current neighbor event is NUD_FAIL and previous
neighbor event is null, which means this neighbor never respond
to the address resolution.

Bug: 286147680
Test: atest NetworkStackTests
Test: atest NetworkStackRootTests
Change-Id: If1b938a5e9af3d3dcb64ead0caa749b3814aed34
1 file changed