netfilter: nf_ct_helper: bail out on duplicated helpers

netfilter: nf_ct_helper: bail out on duplicated helpers

Don't allow registration of helpers using the same tuple:

	{ l3proto, l4proto, src-port }

We lookup for the helper from the packet path using this tuple through
__nf_ct_helper_find(). Therefore, we have to avoid having two helpers
with the same tuple to ensure predictible behaviour.

Don't compare the helper string names anymore since it is valid to
register two helpers with the same name, but using different tuples.
This is also implicitly fixing up duplicated helper registration via
ports= modparam since the name comparison was defeating the tuple
duplication validation.

Reported-by: Feng Gao <gfree.wind@gmail.com>
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>