s390x/3270: handle writes of arbitrary length

The problem is, that the current implementation places unrealistic and
arbitrary constraints on the length of writes to the device (that is the
outbound requests), by asserting ccw.count being such that that even the
worst case escaped payload will fit an  more or less arbitrary sized
buffer. Actually on protocol level there is nothing to justify such
a limitation.

Another strange thing is the return value which more or less reflects
the size (written) after escaping instead of before escaping. This
is strange, because this return value is used to calculate SCSW.count.

Let us teach 3270 how to deal with arbitrary long writes.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reported-by: Jason J . Herne <jjherne@linux.vnet.ibm.com>
Tested-by: Jason J . Herne <jjherne@linux.vnet.ibm.com>
Message-Id: <20170920172314.102710-3-pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
