Eventually, I find out the handler function for inputting route lookup
fail: ip_error().

The drop reasons we used in ip_error() are almost corresponding to
IPSTATS_MIB_*, and following new reasons are introduced:

SKB_DROP_REASON_IP_INADDRERRORS
SKB_DROP_REASON_IP_INNOROUTES

Isn't the name SKB_DROP_REASON_IP_HOSTUNREACH and
SKB_DROP_REASON_IP_NETUNREACH more accurate? To make them corresponding
to IPSTATS_MIB_*, we keep their name still.
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Reviewed-by: Jiang Biao <benbjiang@tencent.com>
Reviewed-by: Hao Peng <flyingpeng@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>