block: don't hold q->sysfs_lock in elevator_init_mq

The original comment says:

	q->sysfs_lock must be held to provide mutual exclusion between
	elevator_switch() and here.

Which is simply wrong. elevator_init_mq() is only called from
blk_mq_init_allocated_queue, which is always called before the request
queue is registered via blk_register_queue(), for dm-rq or normal rq
based driver. However, queue's kobject is only exposed and added to sysfs
in blk_register_queue(). So there isn't such race between elevator_switch()
and elevator_init_mq().

So avoid to hold q->sysfs_lock in elevator_init_mq().

Cc: Christoph Hellwig <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Mike Snitzer <[email protected]>
Cc: Bart Van Assche <[email protected]>
Cc: Damien Le Moal <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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