block: don't protect generic_make_request_checks with blk_queue_enter

Now a063057d ("block: Fix a race between request queue removal and
the block cgroup controller") has been reverted, and blkcg_exit_queue()
won't be called in blk_cleanup_queue() any more.

So don't need to protect generic_make_request_checks() with
blk_queue_enter(), then the total mess can be cleaned.

37f9579f ("blk-mq: Avoid that submitting a bio concurrently with device
removal triggers a crash") is reverted.

Cc: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>