block: Make q_usage_counter also track legacy requests

block: Make q_usage_counter also track legacy requests

This patch makes it possible to pause request allocation for
the legacy block layer by calling blk_mq_freeze_queue() and
blk_mq_unfreeze_queue().

Signed-off-by: Ming Lei <[email protected]>
[ bvanassche: Combined two patches into one, edited a comment and made sure
  REQ_NOWAIT is handled properly in blk_old_get_request() ]
Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Tested-by: Martin Steigerwald <[email protected]>
Tested-by: Oleksandr Natalenko <[email protected]>
Cc: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>