ipv4: Restore accept_local behaviour in fib_validate_source()

ipv4: Restore accept_local behaviour in fib_validate_source()

Commit 7a9bc9b81a5b ("ipv4: Elide fib_validate_source() completely when possible.")
introduced a short-circuit to avoid calling fib_validate_source when not
needed. That change took rp_filter into account, but not accept_local.
This resulted in a change of behaviour: with rp_filter and accept_local
off, incoming packets with a local address in the source field should be
dropped.

Here is how to reproduce the change pre/post 7a9bc9b81a5b commit:
-configure the same IPv4 address on hosts A and B.
-try to send an ARP request from B to A.
-The ARP request will be dropped before that commit, but accepted and answered
after that commit.

This adds a check for ACCEPT_LOCAL, to maintain full
fib validation in case it is 0. We also leave __fib_validate_source() earlier
when possible, based on the same check as fib_validate_source(), once the
accept_local stuff is verified.

Cc: Gregory Detal <[email protected]>
Cc: Christoph Paasch <[email protected]>
Cc: Hannes Frederic Sowa <[email protected]>
Cc: Sergei Shtylyov <[email protected]>
Signed-off-by: Sébastien Barré <[email protected]>
Signed-off-by: David S. Miller <[email protected]>