zfs_get_bootonce
if (zfs_get_bootonce(&currdev, OS_BOOTONCE, bootonce,
if (zfs_get_bootonce(bdev, OS_BOOTONCE, cmd,
if (zfs_get_bootonce(&new_currdev, OS_BOOTONCE_USED,
int zfs_get_bootonce(void *, const char *, char *, size_t);