netfilter: nf_conntrack_sip: de-static helper pointers

Helper's ->help hook can run concurrently with itself, so iterating over
SIP helpers with static pointer won't work reliably.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>