gfs2: Add support for statx inode flags

Add support for the STATX_ATTR_ flags in statx.  (Compression,
encryption, and the nodump flag are not supported by gfs2.)

Partially fixes xfstest generic/424.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>