netfilter: nf_tables: disable preemption when restoring chain counters

With CONFIG_DEBUG_PREEMPT=y

[22144.496057] BUG: using smp_processor_id() in preemptible [00000000] code: iptables-compat/10406
[22144.496061] caller is debug_smp_processor_id+0x17/0x1b
[22144.496065] CPU: 2 PID: 10406 Comm: iptables-compat Not tainted 3.19.0-rc4+ #
[...]
[22144.496092] Call Trace:
[22144.496098]  [<ffffffff8145b9fa>] dump_stack+0x4f/0x7b
[22144.496104]  [<ffffffff81244f52>] check_preemption_disabled+0xd6/0xe8
[22144.496110]  [<ffffffff81244f90>] debug_smp_processor_id+0x17/0x1b
[22144.496120]  [<ffffffffa07c557e>] nft_stats_alloc+0x94/0xc7 [nf_tables]
[22144.496130]  [<ffffffffa07c73d2>] nf_tables_newchain+0x471/0x6d8 [nf_tables]
[22144.496140]  [<ffffffffa07c5ef6>] ? nft_trans_alloc+0x18/0x34 [nf_tables]
[22144.496154]  [<ffffffffa063c8da>] nfnetlink_rcv_batch+0x2b4/0x457 [nfnetlink]

Reported-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>