scsi: libsas: Add missing annotation for sas_ata_qc_issue()

Sparse reports a warning at sas_ata_qc_issue()

warning: context imbalance in sas_ata_qc_issue() - unexpected unlock
The root cause is the missing annotation at sas_ata_qc_issue()

Add the missing __must_hold(ap->lock) annotation

Link: https://lore.kernel.org/r/20200411001933.10072-7-jbi.octave@gmail.com
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libsas/sas_ata.c

@@ -160,6 +160,7 @@ qc_already_gone:
 }
 
 static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
+	__must_hold(ap->lock)
 {
 	struct sas_task *task;
 	struct scatterlist *sg;