net: ip: make fib_validate_source() support drop reasons

In this commit, we make fib_validate_source() and __fib_validate_source()
return -reason instead of errno on error.

The return value of fib_validate_source can be -errno, 0, and 1. It's hard
to make fib_validate_source() return drop reasons directly.

The fib_validate_source() will return 1 if the scope of the source(revert)
route is HOST. And the __mkroute_input() will mark the skb with
IPSKB_DOREDIRECT in this case (combine with some other conditions). And
then, a REDIRECT ICMP will be sent in ip_forward() if this flag exists. We
can't pass this information to __mkroute_input if we make
fib_validate_source() return drop reasons.

Therefore, we introduce the wrapper fib_validate_source_reason() for
fib_validate_source(), which will return the drop reasons on error.

In the origin logic, LINUX_MIB_IPRPFILTER will be counted if
fib_validate_source() return -EXDEV. And now, we need to adjust it by
checking "reason == SKB_DROP_REASON_IP_RPFILTER". However, this will take
effect only after the patch "net: ip: make ip_route_input_noref() return
drop reasons", as we can't pass the drop reasons from
fib_validate_source() to ip_rcv_finish_core() in this patch.

Following new drop reasons are added in this patch:

  SKB_DROP_REASON_IP_LOCAL_SOURCE
  SKB_DROP_REASON_IP_INVALID_SOURCE

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>