mmc: stop abusing the request queue_lock pointer

mmc uses the block layer struct request pointer to indirect their own
lock to the mmc_queue structure, given that the original lock isn't
reachable outside of block.c.  Add a lock pointer to struct mmc_queue
instead and stop overriding the block layer lock which protects fields
entirely separate from the mmc use.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>