blk-mq: remove alloc_hctx and free_hctx methods

There is no need for drivers to control hardware context allocation
now that we do the context to node mapping in common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-mq.c
Simple merge

diff --cc drivers/block/null_blk.c
Simple merge

diff --cc drivers/block/virtio_blk.c
Simple merge

diff --cc include/linux/blk-mq.h
Simple merge