Start the arm64 pmap_pte at level 0

[andrew]

Summary

Return early when the level 0 page table is invalid. This will allows
us to skip over the entire memory range when walking the page tables.

[alc]

I'm confused. I don't see how this change makes any functional difference. The original code set *level == 0 and returned NULL when the L0 entry was not L0_TABLE, and only under those circumstances.