generic: _require_dm_target() helper

generic: _require_dm_target() helper

generic/085 was failing on a machine w/o devicemapper kernel
support because it requires the linear target, but didn't
explicitly test for it.

I could have cut & pasted _require_dm_linear(), but chose
to go the route of a generic helper, _require_dm_target $FOO,
because some day someone will need the zero target, the error
target, or who knows.

Add the helper, use it in test generic/085, and convert
_require_dm_flakey, _require_dm_snapshot, and
_dmerror_required with this new helper.

Reported-by: Angelo Dureghello <angelo.dureghello@nomovok.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>