bio->bi_blkg will be NULL when the issue of the request
has bypassed the block layer as shown in the following oops:

 Internal error: Oops: 96000005 [#1] SMP
 CPU: 17 PID: 2996 Comm: scsi_id Not tainted 5.4.0 #4
 Call trace:
  percpu_counter_add_batch+0x38/0x4c8
  bfqg_stats_update_legacy_io+0x9c/0x280
  bfq_insert_requests+0xbac/0x2190
  blk_mq_sched_insert_request+0x288/0x670
  blk_execute_rq_nowait+0x140/0x178
  blk_execute_rq+0x8c/0x140
  sg_io+0x604/0x9c0
  scsi_cmd_ioctl+0xe38/0x10a8
  scsi_cmd_blk_ioctl+0xac/0xe8
  sd_ioctl+0xe4/0x238
  blkdev_ioctl+0x590/0x20e0
  block_ioctl+0x60/0x98
  do_vfs_ioctl+0xe0/0x1b58
  ksys_ioctl+0x80/0xd8
  __arm64_sys_ioctl+0x40/0x78
  el0_svc_handler+0xc4/0x270

so ensure its validity before using it.

Fixes: fd41e603 ("bfq-iosched: stop using blkg->stat_bytes and ->stat_ios")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>