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/microblaze/include/asm/pgtable.h

@@ -46,8 +46,6 @@ extern int mem_init_done;
 
 #define swapper_pg_dir ((pgd_t *) NULL)
 
-#define pgtable_cache_init()	do {} while (0)
-
 #define arch_enter_lazy_cpu_mode()	do {} while (0)
 
 #define pgprot_noncached_wc(prot)	prot
@@ -526,11 +524,6 @@ extern unsigned long iopa(unsigned long addr);
 /* Needs to be defined here and not in linux/mm.h, as it is arch dependent */
 #define kern_addr_valid(addr)	(1)
 
-/*
- * No page table caches to initialise
- */
-#define pgtable_cache_init()	do { } while (0)
-
 void do_page_fault(struct pt_regs *regs, unsigned long address,
 		   unsigned long error_code);