md: Use enum for overloaded magic numbers used by mddev->curr_resync

mainline inclusion
from mainline-v6.0-rc1
commit eac58d08
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6JN1I
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eac58d08d4937d2eab8f71c663d98d0759845bde



--------------------------------

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: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>