scsi: sd: Keep the discard mode stable

There is a scenario where a large number of discard commands are issued
when the iscsi initiator connects to the target and then performs a session
rescan operation. There is a time window, most of the commands are in UNMAP
mode, and some discard commands become WRITE SAME with UNMAP.

The discard mode has been negotiated during the SCSI probe. If the mode is
temporarily changed from UNMAP to WRITE SAME with UNMAP, an I/O ERROR may
occur because the target may not implement WRITE SAME with UNMAP. Keep the
discard mode stable to fix this issue.

Signed-off-by: Li Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Martin K. Petersen <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

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