md: use separate work_struct for md_start_sync()

It's a little weird to borrow 'del_work' for md_start_sync(), declare
a new work_struct 'sync_work' for md_start_sync().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230825031622.1530464-2-yukuai1@huaweicloud.com