ipv4/fib_hash.c: fix NULL dereference

Unless I miss a guaranteed relation between between "f" and
"new_fa->fa_info" this patch is required for fixing a NULL dereference
introduced by commit a6501e08 ("[IPV4]
FIB_HASH: Reduce memory needs and speedup lookups") and spotted by the
Coverity checker.

Eric Dumazet says:

	Hum, you are right, kmem_cache_free() doesnt allow a NULL
	object, like kfree() does.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>