jfs: define xtree root and page independently

commit a779ed75

 upstream.

In order to make array bounds checking sane, provide a separate
definition of the in-inode xtree root and the external xtree page.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Tested-by: Manas Ghandat <ghandatmanas@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>