ipv4: export fib_info_update_nh_saddr

Add scope as input argument versus relying on fib_info reference in
fib_nh, and export fib_info_update_nh_saddr.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>