Use IPv6 hop limit from upstream interface.

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
    - manually setting /proc/sys/net/ipv6/<upiface>/conf/hop_limit
      to various value yields RAs with expected hop limit settings
      in downstream RAs
Bug: 32163131
Change-Id: I248154ca9d836318bf21a2971d0884040525d9fc