SDEV_READDIR
ASSERT(cmd == SDEV_LOOKUP || cmd == SDEV_READDIR);
SDEV_UNBLOCK_OTHERS(dv, SDEV_READDIR);
(SDEV_READDIR|SDEV_LGWAITING));
SDEV_BLOCK_OTHERS(ddv, (SDEV_READDIR|SDEV_LGWAITING));
(void) sdev_wait4lookup(ddv, SDEV_READDIR);
(dv->sdev_lookup_flags & SDEV_READDIR)