powerpc/mm: Remove flush_HPTE()

The function flush_HPTE() is used in only one place, the implementation
of DEBUG_PAGEALLOC on ppc32.

It's actually a dup of flush_tlb_page() though it's -slightly- more
efficient on hash based processors.  We remove it and replace it by
a direct call to the hash flush code on those processors and to
flush_tlb_page() for everybody else.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>