[POWERPC] Fix special PTE code for secondary hash bucket

The code for mapping special 4k pages on kernels using a 64kB base
page size was missing the code for doing the RPN (real page number)
manipulation when inserting the hardware PTE in the secondary hash
bucket.  It needs the same code as has already been added to the
code that inserts the HPTE in the primary hash bucket.  This adds it.

Spotted by Ben Herrenschmidt.

Signed-off-by: Paul Mackerras <paulus@samba.org>