netfilter: ctnetlink: fix scheduling while atomic

Caused by call to request_module() while holding nf_conntrack_lock.

Reported-and-tested-by: Kövesdi György <kgy@teledigit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>