[scsi] Retry TEST UNIT READY command

The TEST UNIT READY command is issued automatically when the device is
opened, and is not the result of a command being issued by the caller.
This is required in order that a permanent TEST UNIT READY failure can
be used to identify unusable paths in a multipath SAN device.

Since the TEST UNIT READY command is not part of the caller's command
issuing process, it is not covered by any external retry loops (such
as the main retry loop in sandev_command()).

We must therefore be prepared to retry the TEST UNIT READY command
within the SCSI layer itself.  We retry only the TEST UNIT READY
command so as not to multiply the number of potential retries for
normal commands (which are already retried by sandev_command()).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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