When the MPTCP receive path reach a non fatal fall-back condition, e.g.
when the MPC sockets must fall-back to TCP, the existing code is a little
self-inconsistent: it reports that new data is available - return true -
but sets the MPC flag to the opposite value.

As the consequence read operations in some exceptional scenario may block
unexpectedly.

Address the issue setting the correct MPC read status. Additionally avoid
some code duplication in the fatal fall-back scenario.

Fixes: 9c81be0d ("mptcp: add MP_FAIL response support")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>