Add balance_dirty_pages_ratelimited() to cont_expand_zero()

On the systems, ftruncate() which expand size for FAT became the cause
of OOM.  The cont_expand_zero() filled all memory with dirty pages,
and since disk is very slow, limit of page scanning was exceeded, then
it triggered OOM.

This adds balance_dirty_pages_ratelimited() to avoid filling memory
with dirty pages.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -2246,6 +2246,8 @@ int cont_expand_zero(struct file *file, struct address_space *mapping,
 			goto out;
 		BUG_ON(err != len);
 		err = 0;
+
+		balance_dirty_pages_ratelimited(mapping);
 	}
 
 	/* page covers the boundary, find the boundary offset */