smb: do not test the return value of folio_start_writeback()

In preparation for removing the return value entirely, stop testing it
in smb.

Link: https://lkml.kernel.org/r/20231108204605.745109-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/smb/client/file.c

@@ -2706,8 +2706,7 @@ static void cifs_extend_writeback(struct address_space *mapping,
 			 */
 			if (!folio_clear_dirty_for_io(folio))
 				WARN_ON(1);
-			if (folio_start_writeback(folio))
-				WARN_ON(1);
+			folio_start_writeback(folio);
 
 			*_count -= folio_nr_pages(folio);
 			folio_unlock(folio);
@@ -2742,8 +2741,7 @@ static ssize_t cifs_write_back_from_locked_folio(struct address_space *mapping,
 	int rc;
 
 	/* The folio should be locked, dirty and not undergoing writeback. */
-	if (folio_start_writeback(folio))
-		WARN_ON(1);
+	folio_start_writeback(folio);
 
 	count -= folio_nr_pages(folio);
 	len = folio_size(folio);