rtnetlink: fix fdb notification flags

Commit 3ff661c3

 ("net: rtnetlink notify events for FDB NTF_SELF adds and
deletes") reuses the function nlmsg_populate_fdb_fill() to notify fdb events.
But this function was used only for dump and thus was always setting the
flag NLM_F_MULTI, which is wrong in case of a single notification.

Libraries like libnl will wait forever for NLMSG_DONE.

CC: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>