md/raid10: cleanup skip handling in raid10_sync_request

md/raid10: cleanup skip handling in raid10_sync_request

Skip a sector in raid10_sync_request() when it needs no syncing or no
readable device exists. Current skip handling is unnecessary:

- Use 'skip' label to reissue the next sector instead of return directly
- Complete sync and return 'max_sectors' when multiple sectors are skipped
  due to badblocks

The first is error-prone. For example, commit bc49694a9e8f ("md: pass in
max_sectors for pers->sync_request()") removed redundant max_sector
assignments. Since skip modifies max_sectors, `goto skip` leaves
max_sectors equal to sector_nr after the jump, which is incorrect.

The second causes sync to complete erroneously when no actual sync occurs.
For recovery, recording badblocks and continue syncing subsequent sectors
is more suitable. For resync, just skip bad sectors and syncing subsequent
sectors.

Clean up complex and unnecessary skip code. Return immediately when a
sector should be skipped. Reduce code paths and lower regression risk.

Link: https://lore.kernel.org/linux-raid/20260105110300.1442509-12-linan666@huaweicloud.com
Fixes: bc49694a9e8f ("md: pass in max_sectors for pers->sync_request()")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai@fnnas.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>