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/x86/mm/init_32.c

@@ -395,15 +395,6 @@ repeat:
 
 pte_t *kmap_pte;
 
-static inline pte_t *kmap_get_fixmap_pte(unsigned long vaddr)
-{
-	pgd_t *pgd = pgd_offset_k(vaddr);
-	p4d_t *p4d = p4d_offset(pgd, vaddr);
-	pud_t *pud = pud_offset(p4d, vaddr);
-	pmd_t *pmd = pmd_offset(pud, vaddr);
-	return pte_offset_kernel(pmd, vaddr);
-}
-
 static void __init kmap_init(void)
 {
 	unsigned long kmap_vstart;
@@ -412,28 +403,17 @@ static void __init kmap_init(void)
 	 * Cache the first kmap pte:
 	 */
 	kmap_vstart = __fix_to_virt(FIX_KMAP_BEGIN);
-	kmap_pte = kmap_get_fixmap_pte(kmap_vstart);
+	kmap_pte = virt_to_kpte(kmap_vstart);
 }
 
 #ifdef CONFIG_HIGHMEM
 static void __init permanent_kmaps_init(pgd_t *pgd_base)
 {
-	unsigned long vaddr;
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
-	pmd_t *pmd;
-	pte_t *pte;
+	unsigned long vaddr = PKMAP_BASE;
 
-	vaddr = PKMAP_BASE;
 	page_table_range_init(vaddr, vaddr + PAGE_SIZE*LAST_PKMAP, pgd_base);
 
-	pgd = swapper_pg_dir + pgd_index(vaddr);
-	p4d = p4d_offset(pgd, vaddr);
-	pud = pud_offset(p4d, vaddr);
-	pmd = pmd_offset(pud, vaddr);
-	pte = pte_offset_kernel(pmd, vaddr);
-	pkmap_page_table = pte;
+	pkmap_page_table = virt_to_kpte(vaddr);
 }
 
 void __init add_highpages_with_active_regions(int nid,