afs: Don't wait for page writeback with the page lock held

Drop the page lock before waiting for page writeback.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/write.c

@@ -518,17 +518,16 @@ static int afs_writepages_region(struct address_space *mapping,
 		 */
 		lock_page(page);
 
-		if (page->mapping != mapping) {
+		if (page->mapping != mapping || !PageDirty(page)) {
 			unlock_page(page);
 			put_page(page);
 			continue;
 		}
 
+		if (PageWriteback(page)) {
+			unlock_page(page);
 			if (wbc->sync_mode != WB_SYNC_NONE)
 				wait_on_page_writeback(page);
-
-		if (PageWriteback(page) || !PageDirty(page)) {
-			unlock_page(page);
 			put_page(page);
 			continue;
 		}