md: factor out sync completion update into helper

md: factor out sync completion update into helper

Repeatedly reading 'mddev->recovery' flags in md_do_sync() may introduce
potential risk if this flag is modified during sync, leading to incorrect
offset updates. Therefore, replace direct 'mddev->recovery' checks with
'action'.

Move sync completion update logic into helper md_finish_sync(), which
improves readability and maintainability.

The reshape completion update remains safe as it only updated after
successful reshape when MD_RECOVERY_INTR is not set and 'curr_resync'
equals 'max_sectors'.

Link: https://lore.kernel.org/linux-raid/20260105110300.1442509-9-linan666@huaweicloud.com
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai@fnnas.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>