powerpc: Fix bug in kernel copy of libfdt's fdt_subnode_offset_namelen()

There's currently an off-by-one bug in fdt_subnode_offset_namelen()
which causes it to keep searching after it's finished the subnodes of
the given parent, and into the subnodes of siblings of the original
node which come after it in the tree.  This bug was introduced in
commit ed95d745

 ("powerpc: Update
in-kernel dtc and libfdt to version 1.2.0").

A patch has already been submitted to dtc/libfdt mainline.  We don't
really want to pull in a new upstream version during the 2.6.28 cycle,
but we should still fix this bug, hence this standalone version of the
fix for the in-kernel libfdt.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>