[JFFS2][XATTR] rid unnecessary writing of delete marker.

In the followinf situation, an explicit delete marker is not
necessary, because we can certainlly detect those obsolete
xattr_datum or xattr_ref on next mounting.

- When to delete xattr_datum node.
- When to delete xattr_ref node on removing inode.
- When to delete xattr_ref node on updating xattr.

This patch rids writing delete marker in those situations.

Signed-off-by: KaiGai Kohei <kaigai@ak.jp.nec.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>