powerpc64: fix radix on pseries TLB invalidation

When running in a virtualized environment, TLB invalidations can only
be performed on process scope, as only the hypervisor is allowed to
invalidate a global scope, or else a Program Interrupt is triggered.

Since we are here, also make sure that the register process table
hypercall returns success.

Reviewed by: jhibbits
MFC after: 2 weeks
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31775