fsstress: fallback to block size for min dio size

fsstress: fallback to block size for min dio size

XFS_IOC_DIOINFO is only used for XFS, but fsstress use it to get
DIO aligned size. If XFS_IOC_DIOINFO returns error, then stop
doing any DIO related test (dread/dwrite/aread/awrite etc). That
means we never do DIO related test on other filesystems by fsstress.

The real minimal dio size is really not so important for DIO test
in fsstress. The multiple of real min dio size is fine too. I think
the stat.st_blksize get from stat() system call can be used to be
a fake minimal dio size, if XFS_IOC_DIOINFO fails (not supported).

Note that the equation about d_maxiosz is copied from kernel
XFS_IOC_DIOINFO ioctl source code:

  case XFS_IOC_DIOINFO: {
    ...

    da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
    da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);

    ...
  }

[eguan: update commit log add d_maxiosz reference]

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>