Do not truncate the faulting virtual address to a 2 MiB boundary
before looking up its 4 KiB PTE. The truncation discarded the PTE
index and made the fast fault handler inspect entry zero for every
page in the region, causing unnecessary VM faults and TLB flushes.
Fixes: 65bbba25d214 ("powerpc64: Implement Radix MMU for POWER9 CPUs")
MFC after: 2 weeks
Sponsored by: FreeBSD Foundation, Reliable Computer Systems Lab