Line-accurate confirmation (dormant on stock configs - impact today
is nil because the only registered acdone, vfs_stdac_done
(vfs_init.c:434-436), calls vq_done() which is an explicit TODO
no-op (vfs_quota.c:142-146); no runtime demo possible):
sys/kern/vfs_vfsops.c:126-138 vfs_unmount dispatches VFS_ACDONE(mp)
   BEFORE mp->mnt_op->vfs_unmount; dounmount keeps the mount alive
   whenever the FS refuses (EBUSY, vfs_syscalls.c:1000-1031) yet the
   accounting teardown already ran, and mnt_op->vfs_account is never
   cleared (vfs_quota.c:133) -> post-failure VFS_ACCOUNT calls keep
   dispatching into invalidated trees once the TODO is implemented.
Fix moves VFS_ACDONE to the success path.