md/md-bitmap: remove unnecessary local variable in backlog_store()

Local variable is definied first in the beginning of backlog_store(),
there is no need to define it again.

Fixes: 8c13ab11 ("md/bitmap: don't set max_write_behind if there is no write mostly device")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20230706083727.608914-2-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -2569,8 +2569,6 @@ backlog_store(struct mddev *mddev, const char *buf, size_t len)
 			mddev_destroy_serial_pool(mddev, NULL, false);
 	} else if (backlog && !mddev->serial_info_pool) {
 		/* serial_info_pool is needed since backlog is not zero */
-		struct md_rdev *rdev;
-
 		rdev_for_each(rdev, mddev)
 			mddev_create_serial_pool(mddev, rdev, false);
 	}