common/dmthin: dm-thin lacks DAX support

common/dmthin: dm-thin lacks DAX support

generic/347 currently fails when run in conjunction with the DAX
mount option:

generic/347 72s ... - output mismatch (see
/root/project/xfstests/results//generic/347.out.bad)
    --- tests/generic/347.out	2016-05-12 11:56:32.086618744 -0600
    +++ /root/project/xfstests/results//generic/347.out.bad	2018-01-17
    16:04:33.459348448 -0700
    @@ -1,2 +1,3 @@
     QA output created by 347
    +mount: /mnt/xfstests_scratch: can't read superblock on
    /dev/mapper/thin-vol.
     === completed
    ...
    (Run 'diff -u tests/generic/347.out
    /root/project/xfstests/results//generic/347.out.bad'  to see the entire
    diff)

This is expected because the dm-thin target currently lacks DAX
support.

Just skip this test if we are using DAX.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>