ip: use rtnl_send_check() on flush commands, closes 6962

ip: use rtnl_send_check() on flush commands, closes 6962

function                                             old     new   delta
rtnl_send_check                                        -     160    +160
xrtnl_wilddump_request                                64      66      +2
ipneigh_list_or_flush                                714     706      -8
rtnl_send                                             69       -     -69
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/1 up/down: 162/-77)            Total: 85 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>