Comments in the code document special values used for
mddev->curr_resync. Make this clearer by using an enum to label these
values.

The only functional change is a couple places use the wrong comparison
operator that implied 3 is another special value. They are all
fixed to imply that 3 or greater is an active resync.
Signed-off-by: NLogan Gunthorpe <logang@deltatee.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSong Liu <song@kernel.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>