Commit b5dc5d4 ("block,bfq: Disable writeback throttling") tries to
disable wbt for bfq, it's done by calling wbt_disable_default() in
bfq_init_queue(). However, wbt is still enabled if default elevator is
bfq:

device_add_disk
 elevator_init_mq
  bfq_init_queue
   wbt_disable_default -> done nothing

 blk_register_queue
  wbt_enable_default -> wbt is enabled

Fix the problem by checking elevator name if wbt_enable_default() is
called from blk_register_queue().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>

The helper will be used in later patch to make sure wbt is not enabled
if default io scheduler is bfq.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Currently, if wbt is initialized and then disabled by
wbt_disable_default(), sysfs will still show valid wbt_lat_usec, which
will confuse users that wbt is still enabled.

This patch shows wbt_lat_usec as zero and forbid to set it while wbt
is disabled.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reported-and-tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

…rnel/git/akpm/mm

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# Conflicts:
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# Conflicts:
#	drivers/cpufreq/amd-pstate.c

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