rds: use kmem_cache_zalloc instead of kmem_cache_alloc/memset

Use kmem_cache_zalloc instead of kmem_cache_alloc/memset.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Andy Grover <andy.grover@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>