ipv4: Optimize flow initialization in fib_validate_source().

Like in commit 44713b67
("ipv4: Optimize flow initialization in output route lookup."
we can optimize the on-stack flow setup to only initialize
the members which are actually used.

Otherwise we bzero the entire structure, then initialize
explicitly the first half of it.

Signed-off-by: David S. Miller <davem@davemloft.net>