netfilter: use kvmalloc_array to allocate memory for hashtable

nf_ct_alloc_hashtable is used to allocate memory for conntrack,
NAT bysrc and expectation hashtable. Assuming 64k bucket size,
which means 7th order page allocation, __get_free_pages, called
by nf_ct_alloc_hashtable, will trigger the direct memory reclaim
and stall for a long time, when system has lots of memory stress

so replace combination of __get_free_pages and vzalloc with
kvmalloc_array, which provides a overflow check and a fallback
if no high order memory is available, and do not retry to reclaim
memory, reduce stall

and remove nf_ct_free_hashtable, since it is just a kvfree

Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Wang Li <wangli39@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>