FROMLIST: scsi: core: Add support for reserved tags

Allow SCSI LLDs to allocate reserved tags by passing the BLK_MQ_REQ_RESERVED
flag to blk_mq_alloc_request().

Cc: Hannes Reinecke <hare@suse.de>
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Bug: 204762899
Link: https://lore.kernel.org/linux-scsi/20211103000529.1549411-2-bvanassche@acm.org/T/#u


Change-Id: I784756356702cf3368f1501a028f982519c1837f
Signed-off-by: Bart Van Assche <bvanassche@google.com>