drop_monitor: add missing call to genlmsg_end

Update nlmsg_len field with genlmsg_end to enable userspace processing
using nlmsg_next helper. Also adds error handling.

Signed-off-by: Reiter Wolfgang <wr0112358@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>