bridge/mdb: remove wrong use of NLM_F_MULTI

[ Upstream commit 94a72b3f024fc7e9ab640897a1e38583a470659d ]

NLM_F_MULTI must be used only when a NLMSG_DONE message is sent at the end.
In fact, NLMSG_DONE is sent only at the end of a dump.

Libraries like libnl will wait forever for NLMSG_DONE.

Fixes: 949f1e39 ("bridge: mdb: notify on router port add and del")
CC: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>