mptcp: fix unblocking connect()

Currently unblocking connect() on MPTCP sockets fails frequently.
If mptcp_stream_connect() is invoked to complete a previously
attempted unblocking connection, it will still try to create
the first subflow via __mptcp_socket_create(). If the 3whs is
completed and the 'can_ack' flag is already set, the latter
will fail with -EINVAL.

This change addresses the issue checking for pending connect and
delegating the completion to the first subflow. Additionally
do msk addresses and sk_state changes only when needed.

Fixes: 2303f994

 ("mptcp: Associate MPTCP context with TCP socket")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>