以下の負のカーネル値はどういう意味ですか？

カーネル文書では、負の値は意味がありません。

rp_filter - 整数

    0 - No source validation.
    1 - Strict mode as defined in RFC3704 Strict Reverse Path 
        Each incoming packet is tested against the FIB and if the interface
        is not the best reverse path the packet check will fail.
        By default failed packets are discarded.
    2 - Loose mode as defined in RFC3704 Loose Reverse Path 
        Each incoming packet's source address is also tested against the FIB
        and if the source address is not reachable via any interface
        the packet check will fail.

したがって、マイナス記号は間違いである可能性があります。