hammer2_init_devvp
int hammer2_init_devvp(const char *blkdevs, int rootmount,
error = hammer2_init_devvp(devstr, path == NULL, &devvpl);
int hammer2_init_devvp(struct m_vnode *devvp, hammer2_devvp_list_t *devvpl);
error = hammer2_init_devvp(makefs_devvp, &devvpl);