mm: fix build of split ptlock code

Commit 597d795a2a ('mm: do not allocate page->ptl dynamically, if
spinlock_t fits to long') restructures some allocators that are compiled
even if USE_SPLIT_PTLOCKS arn't used.  It results in compilation
failure:

  mm/memory.c:4282:6: error: 'struct page' has no member named 'ptl'
  mm/memory.c:4288:12: error: 'struct page' has no member named 'ptl'

Add in the missing ifdef.

Fixes: 597d795a2a ('mm: do not allocate page->ptl dynamically, if spinlock_t fits to long')
Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -4271,7 +4271,7 @@ void copy_user_huge_page(struct page *dst, struct page *src,
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
 
-#if ALLOC_SPLIT_PTLOCKS
+#if USE_SPLIT_PTE_PTLOCKS && ALLOC_SPLIT_PTLOCKS
 bool ptlock_alloc(struct page *page)
 {
 	spinlock_t *ptl;