f2fs: fix to avoid NULL pointer dereference

Unable to handle kernel NULL pointer dereference at virtual address 00000000
PC is at f2fs_free_dic+0x60/0x2c8
LR is at f2fs_decompress_pages+0x3c4/0x3e8
f2fs_free_dic+0x60/0x2c8
f2fs_decompress_pages+0x3c4/0x3e8
__read_end_io+0x78/0x19c
f2fs_post_read_work+0x6c/0x94
process_one_work+0x210/0x48c
worker_thread+0x2e8/0x44c
kthread+0x110/0x120
ret_from_fork+0x10/0x18

In f2fs_free_dic(), we can not use f2fs_put_page(,1) to release dic->tpages[i],
as the page's mapping is NULL.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/compress.c

@@ -1137,7 +1137,8 @@ void f2fs_free_dic(struct decompress_io_ctx *dic)
 		for (i = 0; i < dic->cluster_size; i++) {
 			if (dic->rpages[i])
 				continue;
-			f2fs_put_page(dic->tpages[i], 1);
+			unlock_page(dic->tpages[i]);
+			put_page(dic->tpages[i]);
 		}
 		kfree(dic->tpages);
 	}