powerpc/booke: Fix TLB1 entry accounting

It's possible, with per-CPU mappings, for TLB1 indices to get out of sync.
This presents a problem when trying to insert an entry into TLB1 of all
CPUs. Currently that's done by assuming (hoping) that the TLBs are
perfectly synced, and inserting to the same index for all CPUs. However,
with aforementioned private mappings, this can result in overwriting
mappings on the other CPUs.

An example:

CPU0                    CPU1
<setup all mappings>    <idle>
    3 private mappings
  kick off CPU 1
                        initialize shared mappings (3 indices low)
                        Load kernel module, triggers 20 new mappings
  Sync mappings at N-3
                        initialize 3 private mappings.

At this point, CPU 1 has all the correct mappings, while CPU 0 is missing 3
mappings that were shared across to CPU 1. When CPU 0 tries to access
memory in one of the overwritten mappings, it hangs while tripping through
the TLB miss handler. Device mappings are not stored in any page table.

This fixes by introducing a '-1' index for tlb1_write_entry_int(), so each
CPU searches for an available index private to itself.

MFC after: 3 weeks