ext4: defrag not supported with DAX

ext4: defrag not supported with DAX

As of this kernel commit:

commit 73f34a5e2ced ("ext4: online defrag not supported with DAX")

online defrag operations for ext4 are disallowed when the filesystem is
mounted with the DAX option.

This causes several xfstests to fail because they expect the defrag
operation to change the file layout:

ext4/308	 [failed, exit status 1] - output mismatch (see /root/xfstests/results//ext4/308.out.bad)
    --- tests/ext4/308.out	2015-10-02 10:19:36.791795792 -0600
    +++ /root/xfstests/results//ext4/308.out.bad	2016-02-17 16:20:52.330454602 -0700
    @@ -23,659 +23,5 @@
     50f924a5dc9b03609a4577f9f961414b  SCRATCH_MNT/test.10
     Perform compacting
     50f924a5dc9b03609a4577f9f961414b  SCRATCH_MNT/test.10
    -Perform compacting, second pass
    -50f924a5dc9b03609a4577f9f961414b  SCRATCH_MNT/test.10
    -Create file with 20 * 2 fragments
    -wrote 1234/1234 bytes at offset 0
    ...
    (Run 'diff -u tests/ext4/308.out /root/xfstests/results//ext4/308.out.bad'  to see the entire diff)
generic/018 1s ... [failed, exit status 1] - output mismatch (see /root/xfstests/results//generic/018.out.bad)
    --- tests/generic/018.out	2016-02-17 16:02:40.103656140 -0700
    +++ /root/xfstests/results//generic/018.out.bad	2016-02-17 16:20:53.117459173 -0700
    @@ -10,10 +10,6 @@
     After: 1
     Write backwards sync, but contiguous - should defrag to 1 extent
     Before: in_range(5, 10)
    -After: 1
    -Write backwards sync leaving holes - defrag should do nothing
    -Before: 16
    -After: 16
    ...
    (Run 'diff -u tests/generic/018.out /root/xfstests/results//generic/018.out.bad'  to see the entire diff)

Avoid this by skipping over defrag tests if we are using ext4 + DAX.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>