fs_mgr_verity: correct dm_target_spec->next

dm_target_spec->next should be set to the next multiple of 8.
It doesn't matter before because dm_ioctl->target_count is set to 1 so
only the first dm_target_spec will be processed.

Bug: None
Test: check device can boot with dm-verity
Change-Id: I96b9b0307a40b05640db06d1c614c16d8144bb47