blk-mq: simplify reallocation of hw ctxs a bit

blk-mq: simplify reallocation of hw ctxs a bit

blk_mq_alloc_and_init_hctx() has already taken reuse into account, so
no need to do it outside, then we can simplify blk_mq_realloc_hw_ctxs().

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220308073219.91173-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>