This introduces the initial support for msk-level socket error
handling.

Currently subflow socket errors are always ignored. That is almost
correct - I think - because the single subflow status does not
affect the msk-level connection.

There a couple of notable exceptions:
- fallen-back msk
- error at connect time on the MPC subflow
both should be propagated at msk level.

patch 1 implement the relevant infra
patch 2 is actuall a bug fix, for some edge cases that become
apparent testing the above.

I'll push a bunch of pktdrill test case which will cover the above.  

Paolo Abeni (2):
  mptcp: deliver ssk errors to msk
  mptcp: fix poll after shutdown

 net/mptcp/protocol.c | 11 ++++++++++-
 net/mptcp/protocol.h |  4 ++++
 net/mptcp/subflow.c  | 43 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 1 deletion(-)

-- 
2.26.2