mm: pgtable: add shortcuts for accessing kernel PMD and PTE

The powerpc 32-bit implementation of pgtable has nice shortcuts for
accessing kernel PMD and PTE for a given virtual address.  Make these
helpers available for all architectures.

[rppt@linux.ibm.com: microblaze: fix page table traversal in setup_rt_frame()]
  Link: http://lkml.kernel.org/r/20200518191511.GD1118872@kernel.org
[akpm@linux-foundation.org: s/pmd_ptr_k/pmd_off_k/ in various powerpc places]

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Cain <bcain@codeaurora.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Guan Xuetao <gxt@pku.edu.cn>
Cc: Guo Ren <guoren@kernel.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Mark Salter <msalter@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Nick Hu <nickhu@andestech.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rich Felker <dalias@libc.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Link: http://lkml.kernel.org/r/20200514170327.31389-9-rppt@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/sh/mm/cache-sh4.c

@@ -207,9 +207,6 @@ static void sh4_flush_cache_page(void *args)
 	struct page *page;
 	unsigned long address, pfn, phys;
 	int map_coherent = 0;
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
 	pmd_t *pmd;
 	pte_t *pte;
 	void *vaddr;
@@ -223,10 +220,7 @@ static void sh4_flush_cache_page(void *args)
 	if (cpu_context(smp_processor_id(), vma->vm_mm) == NO_CONTEXT)
 		return;
 
-	pgd = pgd_offset(vma->vm_mm, address);
-	p4d = p4d_offset(pgd, address);
-	pud = pud_offset(p4d, address);
-	pmd = pmd_offset(pud, address);
+	pmd = pmd_off(vma->vm_mm, address);
 	pte = pte_offset_kernel(pmd, address);
 
 	/* If the page isn't present, there is nothing to do here. */

arch/sh/mm/kmap.c

@@ -14,9 +14,6 @@
 #include <asm/mmu_context.h>
 #include <asm/cacheflush.h>
 
-#define kmap_get_fixmap_pte(vaddr)                                     \
-	pte_offset_kernel(pmd_offset(pud_offset(p4d_offset(pgd_offset_k(vaddr), (vaddr)), (vaddr)), (vaddr)), vaddr)
-
 static pte_t *kmap_coherent_pte;
 
 void __init kmap_coherent_init(void)
@@ -25,7 +22,7 @@ void __init kmap_coherent_init(void)
 
 	/* cache the first coherent kmap pte */
 	vaddr = __fix_to_virt(FIX_CMAP_BEGIN);
-	kmap_coherent_pte = kmap_get_fixmap_pte(vaddr);
+	kmap_coherent_pte = virt_to_kpte(vaddr);
 }
 
 void *kmap_coherent(struct page *page, unsigned long addr)