tile: add virt_to_kpte() API and clean up and document behavior

We use virt_to_pte(NULL, va) a lot, which isn't very obvious.
I added virt_to_kpte(va) as a more obvious wrapper function,
that also validates the va as being a kernel adddress.

And, I fixed the semantics of virt_to_pte() so that we handle
the pud and pmd the same way, and we now document the fact that
we handle the final pte level differently.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

diff --cc arch/tile/include/asm/mmu_context.h
Simple merge

diff --cc arch/tile/include/asm/page.h
Simple merge

diff --cc arch/tile/kernel/setup.c
Simple merge

diff --cc arch/tile/mm/homecache.c
Simple merge

diff --cc arch/tile/mm/init.c
Simple merge

diff --cc arch/tile/mm/pgtable.c
Simple merge