[NETFILTER]: Fix deadlock condition in conntrack/nat-helpers

[NETFILTER]: Fix deadlock condition in conntrack/nat-helpers

There is a possible deadlock condition with conntrack/nat-helpers:

CPU1:
conntrack-helper:help: lock(private_lock)
ip_conntrack_expect_related: write_lock(ip_conntrack_lock)

CPU2:
nat-core:do_bindings: read_lock(ip_conntrack_lock)
nat-helper:help: lock(private_lock)

The lock in the nat-helper is unneccessary because the expectation
is never changed and is protected by ip_conntrack_lock.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>