[NETFILTER]: nf_conntrack: fix smp_processor_id() in preemptible code warning

Since we're using RCU for the conntrack hash now, we need to avoid
getting preempted or interrupted by BHs while changing the stats.

Fixes warning reported by Tilman Schmidt <tilman@imap.cc> when using
preemptible RCU:

[   48.180297] BUG: using smp_processor_id() in preemptible [00000000] code: ntpdate/3562
[   48.180297] caller is __nf_conntrack_find+0x9b/0xeb [nf_conntrack]
[   48.180297] Pid: 3562, comm: ntpdate Not tainted 2.6.25-rc2-mm1-testing #1
[   48.180297]  [<c02015b9>] debug_smp_processor_id+0x99/0xb0
[   48.180297]  [<fac643a7>] __nf_conntrack_find+0x9b/0xeb [nf_conntrack]

Tested-by: Tilman Schmidt <tilman@imap.cc>
Tested-by: Christian Casteyde <casteyde.christian@free.fr> [Bugzilla #10097]

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>