JFS: make sure right-most xtree pages have header.next set to zero

The xtTruncate code was only doing this for leaf pages.  When a file is
horribly fragmented, we may truncate a file leaving an internal page with
an invalid head.next field, which may cause a stale page to be referenced.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>