[NETFILTER]: nf_conntrack_core: avoid taking nf_conntrack_lock in nf_conntrack_alter_reply

The conntrack is unconfirmed, so we have an exclusive reference, which
means that the write_lock is definitely unneeded. A read_lock used to
be needed for the helper lookup, but since we're using RCU for helpers
now rcu_read_lock is enough.

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