mptcp: remove mptcp_skb_entail_init and clean-up MPTCPHDR_SEQ usage

Since the DSS option is built only in mptcp_skb_entail, set
MPTCPHDR_SEQ here. This allows us to remove mptcp_skb_entail_init.

This patch also removes the MPTCPHDR_SEQ check in mptcp_is_data_fin()
as it is not necessary on the TX path and on the RX path checks are
done in mptcp_parse_options() to make sure DATA FIN comes together with
a DSS mapping.

To fix a compare issue mptcp_is_data_fin() is converter to return a
bool. We do the same for a few other similar functions.

The patch also removes setting MPTCPHDR_SEQ in mptcp_send_fin. It was
added in commit e7fabbc2d4 to fix snd_wnd_test. In the meanwhile
snd_wnd_test was reworked and it seem that is no longer needed.

Depends-on: f5ad780c48e2077 (Fix: Create full new skb for the subflow)
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: pack mptcp flags

Some flags are TX or RX only so reuse thise bits to make space for 5
more TX only flags and to convert MPTCPHDR_JOIN to a proper flag.

Depends-on: 6a91bab7c3dac (Cleanup)
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: create helper functions to deal with DSS options

Move code that creates the DSS option into separate functions to allow
code reuse in both mptcp_skb_entail and mptcp_options_write.

Depends-on: f5ad780c48e2077 (Fix: Create full new skb for the subflow)
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>