mm: consolidate pgtable_cache_init() and pgd_cache_init()

Both pgtable_cache_init() and pgd_cache_init() are used to initialize kmem
cache for page table allocations on several architectures that do not use
PAGE_SIZE tables for one or more levels of the page table hierarchy.

Most architectures do not implement these functions and use __weak default
NOP implementation of pgd_cache_init().  Since there is no such default
for pgtable_cache_init(), its empty stub is duplicated among most
architectures.

Rename the definitions of pgd_cache_init() to pgtable_cache_init() and
drop empty stubs of pgtable_cache_init().

Link: http://lkml.kernel.org/r/1566457046-22637-1-git-send-email-rppt@linux.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Will Deacon <will@kernel.org>		[arm64]
Acked-by: Thomas Gleixner <tglx@linutronix.de>	[x86]
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/include/asm/pgtable-nommu.h

@@ -70,11 +70,6 @@ typedef pte_t *pte_addr_t;
  */
 extern unsigned int kobjsize(const void *objp);
 
-/*
- * No page table caches to initialise.
- */
-#define pgtable_cache_init()	do { } while (0)
-
 /*
  * All 32bit addresses are effectively valid for vmalloc...
  * Sort of meaningless for non-VM targets.

arch/arm/include/asm/pgtable.h

@@ -368,8 +368,6 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 #define HAVE_ARCH_UNMAPPED_AREA
 #define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
 
-#define pgtable_cache_init() do { } while (0)
-
 #endif /* !__ASSEMBLY__ */
 
 #endif /* CONFIG_MMU */