powerpc/moea: Fix moea64 native VA invalidation

Summary:
moea64_insert_pteg_native()'s invalidation only works by happenstance.
The purpose of the shifts and XORs is to extract the VSID in order to
reverse-engineer the lower bits of the VPN. Currently a segment size is 256MB
(2**28), and ADDR_API_SHFT64 is 16, so ADDR_PIDX_SHIFT is equivalent. However,
it's semantically incorrect, in that we don't want to shift by the page shift
size, we want to shift to get to the VSID.

Tested by: bdragon
Differential Revision: https://reviews.freebsd.org/D20467