Only need 1 l3mdev FIB rule. Fix setting NLM_F_EXCL in the nlmsghdr.

Fixes: 1aa6c4f6 ("net: vrf: Add l3mdev rules on first device create")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>