include/linux/pagemap.h: optimise find_subpage for !THP

If THP is disabled, find_subpage() can become a no-op by using
hpage_nr_pages() instead of compound_nr(). hpage_nr_pages() embeds a
check for PageTail, so we can drop the check here.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Cc: Aneesh Kumar K.V <[e-mail]>
Cc: Pankaj Gupta <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/pagemap.h

···
 	if (PageHuge(head))
 		return head;
 
-	VM_BUG_ON_PAGE(PageTail(head), head);
-
-	return head + (index & (compound_nr(head) - 1));
+	return head + (index & (hpage_nr_pages(head) - 1));
 }
 
 struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);