ubi: Fix deadlock caused by recursively holding work_sem

During the processing of the bgt, if the sync_erase() return -EBUSY
or some other error code in __erase_worker(),schedule_erase() called
again lead to the down_read(ubi->work_sem) hold twice and may get
block by down_write(ubi->work_sem) in ubi_update_fastmap(),
which cause deadlock.

          ubi bgt                        other task
 do_work
  down_read(&ubi->work_sem)          ubi_update_fastmap
  erase_worker                         # Blocked by down_read
   __erase_worker                      down_write(&ubi->work_sem)
    schedule_erase
     schedule_ubi_work
      down_read(&ubi->work_sem)

Fix this by changing input parameter @nested of the schedule_erase() to
'true' to avoid recursively acquiring the down_read(&ubi->work_sem).

Also, fix the incorrect comment about @nested parameter of the
schedule_erase() because when down_write(ubi->work_sem) is held, the
@nested is also need be true.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217093


Fixes: 2e8f08de ("ubi: Fix races around ubi_refill_pools()")
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>