libbb/loop.c: set_loop

Sometimes loop device is not ready arter get_free_loop, 
raise "can't setup loop device: No such file or directory".


It will be ok if usleep before "goto open_lfd":

try = xasprintf(LOOP_FORMAT, i); 
for (lc=0; lc<100; lc++) {
  if (stat(try, &buf2)==0) break; 
  usleep(20);
}

goto open_lfd;