md: use separate work_struct for md_start_sync()

[ Upstream commit ac619781 ]

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


Stable-dep-of: 8d28d0dd ("md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime")
Signed-off-by: Sasha Levin <sashal@kernel.org>