scsi: iscsi_tcp: set BDI_CAP_STABLE_WRITES when data digest enabled

scsi: iscsi_tcp: set BDI_CAP_STABLE_WRITES when data digest enabled

iscsi tcp will first send out data, then calculate and send data
digest. If we don't have BDI_CAP_STABLE_WRITES, the page cache will be
written in spite of the on going writeback. Consequently, wrong digest
will be got and sent to target.

To fix this, set BDI_CAP_STABLE_WRITES when data digest is enabled
in iscsi_tcp .slave_configure callback.

Signed-off-by: Jianchao Wang <[email protected]>
Acked-by: Chris Leech <[email protected]>
Acked-by: Lee Duncan <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>