switchdev: add netlink flags to IPv4 FIB add op

Pass in the netlink flags (NLM_F_*) into switchdev driver for IPv4 FIB add op
to allow driver to 1) optimize hardware updates, 2) handle ip route prepend
and append commands correctly.

Suggested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Suggested-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>