netfilter: nfnetlink_cthelper: use {READ,WRITE}_ONCE for accessing helper flags

netfilter: nfnetlink_cthelper: use {READ,WRITE}_ONCE for accessing helper flags

Conntrack helper flags are accessed from packet and netlink dump path.
Concurrent update of userspace helper flags is not possible, because the
nfnl_mutex in held on updates. These flags are only used by userspace
helpers. Use {READ,WRITE}_ONCE() to access this flags from lockless
paths.

Fixes: 12f7a505331e ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>