mptcp: annotate lockless accesses around read-mostly fields

The following MPTCP socket fields:

 - can_ack
 - fully_established
 - rcv_data_fin
 - snd_data_fin_enable
 - rcv_fastclose
 - use_64bit_ack

are accessed without any lock, add the appropriate annotation.

The schema is safe as each field can change its value at most
once in the whole mptcp socket life cycle.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>