[SCSI] qla2xxx: Correct residual-count handling discrepancies during UNDERRUN handling.

For recent ISPs, software during CS_UNDERRUN handling must
determine if the two residuals, firmware-calculated and FCP_RSP,
are different to recognize if a frame has been dropped. Update
the driver to catch this condition, and clear the
SS_RESIDUAL_UNDER and lscsi_status bits. This logic is
consistent with what earlier firmwares did by explicitly
cracking open the FCP_RSP statuses and clearing
SS_RESIDUAL_UNDER.

Signed-off-by: Andrew Vasquez <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/qla2xxx/qla_isr.c

···
 	case CS_DATA_UNDERRUN:
 		resid = resid_len;
 		/* Use F/W calculated residual length. */
-		if (IS_FWI2_CAPABLE(ha))
+		if (IS_FWI2_CAPABLE(ha)) {
+			if (scsi_status & SS_RESIDUAL_UNDER &&
+			    resid != fw_resid_len) {
+				scsi_status &= ~SS_RESIDUAL_UNDER;
+				lscsi_status = 0;
+			}
 			resid = fw_resid_len;
+		}
 
 		if (scsi_status & SS_RESIDUAL_UNDER) {
 			scsi_set_resid(cp, resid);