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.comReviewed-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>