xfstests: add _require_freeze and minor cleanups

xfstests: add _require_freeze and minor cleanups

More filesystems have grown freeze capability, so rather than
hardcoding several in _supported_fs, make tests 068 and 280
generic and then add a new _require_freeze() which checks whether
the fs under test can be frozen before beginning the test.

Minor other cleanups to 280:
- remove extra _supported_fs line
- clear $seq.full before beginning

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>