f2fs: fix to enlarge size of write_io_dummy mempool

It needs to double cache size of write_io_dummy mempool, otherwise we may
run out of cache in scenraio of Data/Node IOs were issued concurrently.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1929,7 +1929,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 
 	if (F2FS_IO_SIZE(sbi) > 1) {
 		sbi->write_io_dummy =
-			mempool_create_page_pool(F2FS_IO_SIZE(sbi) - 1, 0);
+			mempool_create_page_pool(2 * (F2FS_IO_SIZE(sbi) - 1), 0);
 		if (!sbi->write_io_dummy)
 			goto free_options;
 	}