ip6mr: plug drop_reason to ip6mr_cache_report()

ip6mr: plug drop_reason to ip6mr_cache_report()

- Check mrt->mroute_sk earlier in the function.

- Use sock_queue_rcv_skb_reason() instead of sock_queue_rcv_skb().
- Use sk_skb_reason_drop() instead of kfree_skb().
  Note that we return -ENOMEM if sock_queue_rcv_skb_reason() failed,
  as the precise error is not really needed for callers.

- Remove one net_warn_ratelimited().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260430074004.4133602-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>