md: fix missing flush of sync_work

md: fix missing flush of sync_work

Commit ac619781967b ("md: use separate work_struct for md_start_sync()")
use a new sync_work to replace del_work, however, stop_sync_thread() and
__md_stop_writes() was trying to wait for sync_thread to be done, hence
they should switch to use sync_work as well.

Noted that md_start_sync() from sync_work will grab 'reconfig_mutex',
hence other contex can't held the same lock to flush work, and this will
be fixed in later patches.

Fixes: ac619781967b ("md: use separate work_struct for md_start_sync()")
Signed-off-by: Yu Kuai <[email protected]>
Acked-by: Xiao Ni <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]