f2fs: fix compile warnings: 'struct *' declared inside parameter list

We meet these compile warnings below, which caused by missing declare structs:
struct f2fs_io_info, struct extent, struct f2fs_sb_info.

warning: 'struct f2fs_io_info' declared inside parameter list
warning: 'struct extent_info' declared inside parameter list
warning: 'struct f2fs_sb_info' declared inside parameter list

Signed-off-by: Zhikang Zhang <zhangzhikang1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>