mm/shmem: fix build without THP

Some optimizers don't notice that shmem_punch_compound() is always true
(PageTransCompound() being false) without CONFIG_TRANSPARENT_HUGEPAGE==y.

Use IS_ENABLED to help them to avoid the BUILD_BUG inside HPAGE_PMD_NR.

Fixes: 71725ed ("mm: huge tmpfs: try to split_huge_page() when punching hole")
Reported-by: Randy Dunlap <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Randy Dunlap <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: Hugh Dickins

mm/shmem.c

@@ -952,7 +952,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 				VM_BUG_ON_PAGE(PageWriteback(page), page);
 				if (shmem_punch_compound(page, start, end))
 					truncate_inode_page(mapping, page);
-				else {
+				else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 					/* Wipe the page and don't get stuck */
 					clear_highpage(page);
 					flush_dcache_page(page);