netfilter: nf_conntrack_helper: cap maximum number of expectation at helper registration

netfilter: nf_conntrack_helper: cap maximum number of expectation at helper registration

On helper registration, the maximum number of expectations cannot go over
NF_CT_EXPECT_MAX_CNT (255), but zero can be specified then
nf_conntrack_expect_max applies. Turn zero into NF_CT_EXPECT_MAX_CNT
otherwise, expectation LRU eviction on insertion is disabled.

Moreover, expand this sanity check all expectation classes.

This max_expecy policy is only tunable since userspace helpers are
available, set Fixes: tag to the commit that adds such infrastructure.

Remove the check for p->max_expected given this field must always
be non-zero after this patch.

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