shmem: Fix link error if huge pages support is disabled

If CONFIG_TRANSPARENT_HUGE_PAGECACHE=n, HPAGE_PMD_NR evaluates to
BUILD_BUG_ON(), and may cause (e.g. with gcc 4.12):

    mm/built-in.o: In function `shmem_alloc_hugepage':
    shmem.c:(.text+0x17570): undefined reference to `__compiletime_assert_1365'

To fix this, move the assignment to hindex after the check for huge
pages support.

Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1362,13 +1362,14 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
 	struct vm_area_struct pvma;
 	struct inode *inode = &info->vfs_inode;
 	struct address_space *mapping = inode->i_mapping;
-	pgoff_t idx, hindex = round_down(index, HPAGE_PMD_NR);
+	pgoff_t idx, hindex;
 	void __rcu **results;
 	struct page *page;
 
 	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
 		return NULL;
 
+	hindex = round_down(index, HPAGE_PMD_NR);
 	rcu_read_lock();
 	if (radix_tree_gang_lookup_slot(&mapping->page_tree, &results, &idx,
 				hindex, 1) && idx < hindex + HPAGE_PMD_NR) {