Bio attribution is handled at bio_set_dev() as once we have a device, we
have a corresponding request_queue and then can derive the current css.
In special cases, we want to attribute to bio to someone else. This can
be done by calling bio_associate_blkg_from_css() or
kthread_associate_blkcg() depending on the scenario. Btrfs does this for
compressed writeback as they are handled by kworkers, so the latter can
be done here.

Commit 1a41802701ec ("btrfs: drop bio_set_dev where not needed") removes
early bio_set_dev() calls prior to submit_stripe_bio(). This breaks the
above assumption that we'll have a request_queue when we are doing
association. To fix this, switch to using kthread_associate_blkcg().

Without this, we crash in btrfs/024:

  [ 3052.093088] BUG: kernel NULL pointer dereference, address: 0000000000000510
  [ 3052.107013] #PF: supervisor read access in kernel mode
  [ 3052.107014] #PF: error_code(0x0000) - not-present page
  [ 3052.107015] PGD 0 P4D 0
  [ 3052.107021] Oops: 0000 [#1] SMP
  [ 3052.138904] CPU: 42 PID: 201270 Comm: kworker/u161:0 Kdump: loaded Not tainted 5.5.0-rc1-00062-g4852d8ac90a9 #712
  [ 3052.138905] Hardware name: Quanta Tioga Pass Single Side 01-0032211004/Tioga Pass Single Side, BIOS F08_3A18 12/20/2018
  [ 3052.138912] Workqueue: btrfs-delalloc btrfs_work_helper
  [ 3052.191375] RIP: 0010:bio_associate_blkg_from_css+0x1e/0x3c0
  [ 3052.191379] RSP: 0018:ffffc900210cfc90 EFLAGS: 00010282
  [ 3052.191380] RAX: 0000000000000000 RBX: ffff88bfe5573c00 RCX: 0000000000000000
  [ 3052.191382] RDX: ffff889db48ec2f0 RSI: ffff88bfe5573c00 RDI: ffff889db48ec2f0
  [ 3052.191386] RBP: 0000000000000800 R08: 0000000000203bb0 R09: ffff889db16b2400
  [ 3052.293364] R10: 0000000000000000 R11: ffff88a07fffde80 R12: ffff889db48ec2f0
  [ 3052.293365] R13: 0000000000001000 R14: ffff889de82bc000 R15: ffff889e2b7bdcc8
  [ 3052.293367] FS:  0000000000000000(0000) GS:ffff889ffba00000(0000) knlGS:0000000000000000
  [ 3052.293368] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 3052.293369] CR2: 0000000000000510 CR3: 0000000002611001 CR4: 00000000007606e0
  [ 3052.293370] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  [ 3052.293371] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  [ 3052.293372] PKRU: 55555554
  [ 3052.293376] Call Trace:
  [ 3052.402552]  btrfs_submit_compressed_write+0x137/0x390
  [ 3052.402558]  submit_compressed_extents+0x40f/0x4c0
  [ 3052.422401]  btrfs_work_helper+0x246/0x5a0
  [ 3052.422408]  process_one_work+0x200/0x570
  [ 3052.438601]  ? process_one_work+0x180/0x570
  [ 3052.438605]  worker_thread+0x4c/0x3e0
  [ 3052.438614]  kthread+0x103/0x140
  [ 3052.460735]  ? process_one_work+0x570/0x570
  [ 3052.460737]  ? kthread_mod_delayed_work+0xc0/0xc0
  [ 3052.460744]  ret_from_fork+0x24/0x30

Fixes: 1a41802701ec ("btrfs: drop bio_set_dev where not needed")
Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>