block: fix race between wbt_enable_default and IO submission

block: fix race between wbt_enable_default and IO submission

When wbt_enable_default() is moved out of queue freezing in elevator_change(),
it can cause the wbt inflight counter to become negative (-1), leading to hung
tasks in the writeback path. Tasks get stuck in wbt_wait() because the counter
is in an inconsistent state.

The issue occurs because wbt_enable_default() could race with IO submission,
allowing the counter to be decremented before proper initialization. This manifests
as:

  rq_wait[0]:
    inflight:             -1
    has_waiters:        True

rwb_enabled() checks the state, which can be updated exactly between wbt_wait()
(rq_qos_throttle()) and wbt_track()(rq_qos_track()), then the inflight counter
will become negative.

And results in hung task warnings like:
  task:kworker/u24:39 state:D stack:0 pid:14767
  Call Trace:
    rq_qos_wait+0xb4/0x150
    wbt_wait+0xa9/0x100
    __rq_qos_throttle+0x24/0x40
    blk_mq_submit_bio+0x672/0x7b0
    ...

Fix this by:

1. Splitting wbt_enable_default() into:
   - __wbt_enable_default(): Returns true if wbt_init() should be called
   - wbt_enable_default(): Wrapper for existing callers (no init)
   - wbt_init_enable_default(): New function that checks and inits WBT

2. Using wbt_init_enable_default() in blk_register_queue() to ensure
   proper initialization during queue registration

3. Move wbt_init() out of wbt_enable_default() which is only for enabling
   disabled wbt from bfq and iocost, and wbt_init() isn't needed. Then the
   original lock warning can be avoided.

4. Removing the ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT flag and its handling
   code since it's no longer needed

This ensures WBT is properly initialized before any IO can be submitted,
preventing the counter from going negative.

Cc: Nilay Shroff <nilay@linux.ibm.com>
Cc: Yu Kuai <yukuai@fnnas.com>
Cc: Guangwu Zhang <guazhang@redhat.com>
Fixes: 78c271344b6f ("block: move wbt_enable_default() out of queue freezing from sched ->exit()")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>