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>

arch/tile/include/asm/mmu_context.h

@@ -45,7 +45,7 @@ static inline void __install_page_table(pgd_t *pgdir, int asid, pgprot_t prot)
 
 static inline void install_page_table(pgd_t *pgdir, int asid)
 {
-	pte_t *ptep = virt_to_pte(NULL, (unsigned long)pgdir);
+	pte_t *ptep = virt_to_kpte((unsigned long)pgdir);
 	__install_page_table(pgdir, asid, *ptep);
 }
 

arch/tile/include/asm/page.h

@@ -328,6 +328,7 @@ static inline int pfn_valid(unsigned long pfn)
 
 struct mm_struct;
 extern pte_t *virt_to_pte(struct mm_struct *mm, unsigned long addr);
+extern pte_t *virt_to_kpte(unsigned long kaddr);
 
 #endif /* !__ASSEMBLY__ */