md: rename recovery_cp to resync_offset

'recovery_cp' was used to represent the progress of sync, but its name
contains recovery, which can cause confusion. Replaces 'recovery_cp'
with 'resync_offset' for clarity.

Signed-off-by: Li Nan <linan122@huawei.com>
Link: https://lore.kernel.org/linux-raid/20250722033340.1933388-1-linan666@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>