blk-mq: bump max tag depth to 10K tags

For some scsi-mq cases, the tag map can be huge. So increase the
max number of tags we support.

Additionally, don't fail with EINVAL if a user requests too many
tags. Warn that the tag depth has been adjusted down, and store
the new value inside the tag_set passed in.

Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-mq.h

@@ -135,7 +135,7 @@ enum {
 	BLK_MQ_S_STOPPED	= 0,
 	BLK_MQ_S_TAG_ACTIVE	= 1,
 
-	BLK_MQ_MAX_DEPTH	= 2048,
+	BLK_MQ_MAX_DEPTH	= 10240,
 
 	BLK_MQ_CPU_WORK_BATCH	= 8,
 };