s390/dasd: add copy pair swap capability

In case of errors or misbehaviour of the primary device a controlled
failover to one of the configured secondary devices needs to be
performed.

The swap processing stops I/O on the primary device, all requests are
re-queued to the blocklayer queue, the entries in the copy relation are
swapped and finally the link to the blockdevice is moved from primary to
secondary dasd device.
After this, the secondary becomes the new primary device and I/O is
restarted on that device.

Signed-off-by: Stefan Haberland <[email protected]>
Reviewed-by: Jan Hoeppner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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