Questions about XFS abnormal img mount test

["zhengbin (A)" <zhengbin13@huawei.com>]

### question
We recently used fuzz(hydra) to test 4.19 stable XFS and automatically generate tmp.img (XFS v5 format, but some metadata is wrong)

Test as follows:
mount tmp.img tmpdir
cp file tmpdir
sync  --> stuck

### cause analysis
This is because tmp.img (only 1 AG) has some problems. Using xfs_repair detect information as follows:

agf_freeblks 0, counted 3224 in ag 0
agf_longest 536874136, counted 3224 in ag 0 
sb_fdblocks 613, counted 3228

The reason sync is blocked is :
xfs_vm_writepages(xfs_address_space_operations--writepages)
  write_cache_pages
    xfs_do_writepage
      xfs_writepage_map
	xfs_map_blocks
          allocate_blocks:
	    error = xfs_iomap_write_allocate
			
xfs_iomap_write_allocate
  while (count_fsb != 0) {
    nimaps = 0;
      while (nimaps == 0) { --> endless loop
	nimaps = 1;
	error = xfs_bmapi_write(..., &nimaps) --> nimaps becomes 0 again

xfs_bmapi_write
  xfs_bmap_alloc
    xfs_bmap_btalloc
      xfs_alloc_vextent
	xfs_alloc_fix_freelist
          xfs_alloc_space_available --> less space than needed

xfs_alloc_space_available
  alloc_len = args->minlen + (args->alignment - 1) + args->minalignslop;
    longest = xfs_alloc_longest_free_extent(pag, min_free, reservation);
    if (longest < alloc_len)
       return false;

    /* do we have enough free space remaining for the allocation? */
    available = (int)(pag->pagf_freeblks + pag->pagf_flcount -
                        reservation - min_free - args->minleft);
    if (available < (int)max(args->total, alloc_len))
      return false;

### solve
1. Detect the above metadata corruption when mounting XFS?
   agf_freeblks 0, counted 3224 in ag 0
   agf_longest 536874136, counted 3224 in ag 0 
   sb_fdblocks 613, counted 3228

2. xfs_repair detection at system boot? If xfs_repair fails, refuse to mount XFS