powerpc/booke: Do as much work outside of TLB locks as possible

Reduce the surface area of the TLB locks. Unfortunately the same trick for
serializing the tlbie instruction on OEA64 cannot be used here to reduce the
scope of the tlbivax mutex to the tlbsync only, as the mutex also serializes
the TLB miss lock as a side effect, so contention on this lock may not be
reducible any further.