md: keep recovery_cp in mdp_superblock_s

commit 907a99c3 ("md: rename recovery_cp to resync_offset") replaces
recovery_cp with resync_offset in mdp_superblock_s which is in md_p.h.
md_p.h is used in userspace too. So mdadm building fails because of this.
This patch revert this change.

Fixes: 907a99c3 ("md: rename recovery_cp to resync_offset")
Signed-off-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/linux-raid/20250815040028.18085-1-xni@redhat.com


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