target-ppc: Return page shift from PTEG search

ppc_hash64_pteg_search() now decodes a PTEs page size encoding, which it
didn't previously do.  This means we're now double decoding the page size
because we check it int he fault path after ppc64_hash64_htab_lookup()
returns.

To avoid this duplication have ppc_hash64_pteg_search() and
ppc_hash64_htab_lookup() return the page size from the PTE and use that in
the callers instead of decoding again.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
