+# Required for for_each_perag*
+ccflags-y += -std=gnu99

I don't think it is up to an individual subsystem to pick a specific C
dialect.

I think the most important reason why the kernel sticks with gnu89 is
to avoid the misfeature of variable declarations in the middle of
blocks, and this change would lose it.

+	xfs_agnumber_t		last_agno = 0;
 	int			saved_error = 0;
 	int			error = 0;
 	LIST_HEAD		(buffer_list);
 
 	/* update secondary superblocks. */
-	for_each_perag_from(mp, agno, pag) {
+	for_each_perag_from(mp, iter, 1) {
 		struct xfs_buf		*bp;
 
+		last_agno = iter.pag->pag_agno;

This is a really horrible API as it magically injects a local variable
in a macro.  It also leads to worse code generation and a small but
noticable increase in .text sie:

hch@brick:~/work/xfs$ size xfs.o.*
   text	   data	    bss	    dec	    hex	filename
1521421	 301161	   1880	1824462	 1bd6ce	xfs.o.old
1521516	 301161	   1880	1824557	 1bd72d	xfs.o.new