gfs2: Fix xattr fsync

Make sure that changing xattrs marks the corresponding inode dirty so
that a subsequent fsync will sync those changes to disk.  We set
I_DIRTY_SYNC as well as I_DIRTY_DATASYNC so that both fsync and
fdatasync will sync xattr changes: xattrs can contain information
critical to how the data can be accessed, so we don't want fdatasync
to skip them.

Fixes xfstest generic/066.

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