Currently we rely on the subflow->data_avail field, which is subject to
races:

	ssk1
		skb len = 500 DSS(seq=1, len=1000, off=0)
		# data_avail == MPTCP_SUBFLOW_DATA_AVAIL

	ssk2
		skb len = 500 DSS(seq = 501, len=1000)
		# data_avail == MPTCP_SUBFLOW_DATA_AVAIL

	ssk1
		skb len = 500 DSS(seq = 1, len=1000, off =500)
		# still data_avail == MPTCP_SUBFLOW_DATA_AVAIL,
		# as the skb is covered by a pre-existing map,
		# which was in-sequence at reception time.

Instead we can explicitly check if some has been received in-sequence,
propagating the info from __mptcp_move_skbs_from_subflow().

Additionally add the 'ONCE' annotation to the 'data_avail' memory
access, as msk will read it outside the subflow socket lock.

Fixes: 648ef4b8 ("mptcp: Implement MPTCP receive path")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>