af_iucv: sync sk shutdown flag if iucv path is quiesced

If the af_iucv communication partner quiesces the path to shutdown its
receive direction, provide a quiesce callback implementation to shutdown
the (local) send direction. This ensures that both sides are synchronized.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>