mm/shmem.c: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Link: http://lkml.kernel.org/r/20171020191058.GA24427@embeddedor.com
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -4098,6 +4098,7 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
 			if (i_size >= HPAGE_PMD_SIZE &&
 					i_size >> PAGE_SHIFT >= off)
 				return true;
+			/* fall through */
 		case SHMEM_HUGE_ADVISE:
 			/* TODO: implement fadvise() hints */
 			return (vma->vm_flags & VM_HUGEPAGE);