mm: thp: microoptimize compound_mapcount()

compound_mapcount() is only called after PageCompound() has already been
checked by the caller, so there's no point to check it again.  Gcc may
optimize it away too because it's inline but this will remove the
runtime check for sure and add it'll add an assert instead.

Link: http://lkml.kernel.org/r/1462547040-1737-3-git-send-email-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -475,8 +475,7 @@ static inline atomic_t *compound_mapcount_ptr(struct page *page)
 
 static inline int compound_mapcount(struct page *page)
 {
-	if (!PageCompound(page))
-		return 0;
+	VM_BUG_ON_PAGE(!PageCompound(page), page);
 	page = compound_head(page);
 	return atomic_read(compound_mapcount_ptr(page)) + 1;
 }