bridge/mdb: remove wrong use of NLM_F_MULTI

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

Libraries like libnl will wait forever for NLMSG_DONE.

Fixes: 37a393bc ("bridge: notify mdb changes via netlink")
CC: Cong Wang <amwang@redhat.com>
CC: Stephen Hemminger <stephen@networkplumber.org>
CC: bridge@lists.linux-foundation.org
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>