[IPV4] fib_hash: kmalloc + memset conversion to kzalloc

fib_hash: kmalloc + memset conversion to kzalloc
fix to avoid memset entirely.

Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>