mptcp: fix poll after shutdown

The current mptcp_poll() implementation gives unexpected
results after shutdown(SEND_SHUTDOWN) and when the msk
status is TCP_CLOSE.

Set the correct mask.

Fixes: 8edf0864 ("mptcp: rework poll+nospace handling")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>