netfilter: rework user-space expectation helper support

This partially reworks bc01befd
which added userspace expectation support.

This patch removes the nf_ct_userspace_expect_list since now we
force to use the new iptables CT target feature to add the helper
extension for conntracks that have attached expectations from
userspace.

A new version of the proof-of-concept code to implement userspace
helpers from userspace is available at:

http://people.netfilter.org/pablo/userspace-conntrack-helpers/nf-ftp-helper-POC.tar.bz2



This patch also modifies the CT target to allow to set the
conntrack's userspace helper status flags. This flag is used
to tell the conntrack system to explicitly allocate the helper
extension.

This helper extension is useful to link the userspace expectations
with the master conntrack that is being tracked from one userspace
helper.

This feature fixes a problem in the current approach of the
userspace helper support. Basically, if the master conntrack that
has got a userspace expectation vanishes, the expectations point to
one invalid memory address. Thus, triggering an oops in the
expectation deletion event path.

I decided not to add a new revision of the CT target because
I only needed to add a new flag for it. I'll document in this
issue in the iptables manpage. I have also changed the return
value from EINVAL to EOPNOTSUPP if one flag not supported is
specified. Thus, in the future adding new features that only
require a new flag can be added without a new revision.

There is no official code using this in userspace (apart from
the proof-of-concept) that uses this infrastructure but there
will be some by beginning 2012.

Reported-by: Sam Roberts <vieuxtech@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>