netfilter: ipset: fix ip_set_list allocation failure

ip_set_create() and ip_set_net_init() attempt to allocate physically
contiguous memory for ip_set_list. If memory is fragmented, the
allocations could easily fail:

        vzctl: page allocation failure: order:7, mode:0xc0d0

        Call Trace:
         dump_stack+0x19/0x1b
         warn_alloc_failed+0x110/0x180
         __alloc_pages_nodemask+0x7bf/0xc60
         alloc_pages_current+0x98/0x110
         kmalloc_order+0x18/0x40
         kmalloc_order_trace+0x26/0xa0
         __kmalloc+0x279/0x290
         ip_set_net_init+0x4b/0x90 [ip_set]
         ops_init+0x3b/0xb0
         setup_net+0xbb/0x170
         copy_net_ns+0xf1/0x1c0
         create_new_namespaces+0xf9/0x180
         copy_namespaces+0x8e/0xd0
         copy_process+0xb61/0x1a00
         do_fork+0x91/0x320

Use kvcalloc() to fallback to 0-order allocations if high order
page isn't available.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>