[POWERPC] Fix warning in hpte_decode(), and generalize it

This adds the necessary support to hpte_decode() to handle 1TB
segments and 16GB pages, and removes an uninitialized value
warning on avpn.

We don't have any code to generate HPTEs for 1TB segments or 16GB
pages yet, so this is mostly for completeness, and to fix the
warning.

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