fib: introduce FIB info offload flag helpers

These helpers are to be used in case someone offloads the FIB entry. The
result is that if the entry is offloaded to at least one device, the
offload flag is set.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>