fstests: _require_dm_target should not always skip DAX capable devices

fstests: _require_dm_target should not always skip DAX capable devices

Recent changes have turned off all dm-error, dm-thin and dm-flakey
tests on pmem devices even when we are not explicitly testing DAX.
This is a regression resulting in a large number of log recovery
tests no longer running on my pmem-based test VMs. I added the "-o
dax=never" mount options to these test configs, only to find it
still would not run the dm tests even though the filesystem will
never use DAX.

Fix this so that the dm target DAX test explicitly ignores the
the block device DAX capability when the filesystem is mounted with
dax=never and hence we can use all the dm targets when the tests are
being run with FSDAX disabled.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>