This is alike the previous change, with some additional ipv4 specific
quirk. Even when using the route hint we still have to do perform
additional per packet checks about source address validity: a new
helper is added to wrap them.

Hints are explicitly disabled if the destination is a local broadcast,
that keeps the code simple and local broadcast are a slower path anyway.

UDP flood performances vs recvmmsg() receiver:

vanilla		patched		delta
Kpps		Kpps		%
1683		1871		+11

In the worst case scenario - each packet has a different
destination address - the performance delta is within noise
range.

v3 -> v4:
 - re-enable hints for forward

v2 -> v3:
 - really fix build (sic) and hint usage check
 - use fib4_has_custom_rules() helpers (David A.)
 - add ip_extract_route_hint() helper (Edward C.)
 - use prev skb as hint instead of copying data (Willem)

v1 -> v2:
 - fix build issue with !CONFIG_IP_MULTIPLE_TABLES
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>