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>