Local variable of 'capacity' stores the previous disk capacity, and
'size' variable records the latest disk capacity, so swap them for
fixing logging on capacity change.

Cc: Christoph Hellwig <hch@lst.de>
Fixes: a782483c ("block: remove the nr_sects field in struct hd_struct")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>