s390/AP: support new dynamic AP bus size limit

s390/AP: support new dynamic AP bus size limit

This patch provides support for new dynamic AP bus message limit
with the existing zcrypt device driver and AP bus core code.

There is support for a new field 'ml' from TAPQ query. The field
gives if != 0 the AP bus limit for this card in 4k chunk units.
The actual message size limit per card is shown as a new read-only
sysfs attribute. The sysfs attribute

  /sys/devices/ap/cardxx/max_msg_size

shows the upper limit in bytes used by the AP bus and zcrypt device
driver for requests and replies send to and received from this card.
Currently up to CEX7 support only max 12kB msg size and thus the field
shows 12288 meaning the upper limit of a valid msg for this card is
12kB. Please note that the usable payload is somewhat lower and
depends on the msg type and thus the header struct which is to be
prepended by the zcrypt dd.

The dispatcher responsible for choosing the right card and queue is
aware of the individual card AP bus message limit. So a request is
only assigned to a queue of a card which is able to handle the size of
the request (e.g. a 14kB request will never go to a max 12kB card).
If no such card is found the ioctl will fail with ENODEV.

The reply buffer held by the device driver is determined by the ml
field of the TAPQ for this card. If a response from the card exceeds
this limit however, the response is not truncated but the ioctl for
this request will fail with errno EMSGSIZE to indicate that the device
driver has dropped the response because it would overflow the buffer
limit.

If the request size does not indicate to the dispatcher that an
adapter with extended limit is to be used, a random card will be
chosen when no specific card is addressed (ANY addressing). This may
result in an ioctl failure when the reply size needs an adapter with
extended limit but the randomly chosen one is not capable of handling
the broader reply size. The user space application needs to use
dedicated addressing to forward such a request only to suitable cards
to get requests like this processed properly.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Tuchscherer <ingo.tuchscherer@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>