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: NZhikang Zhang <zhangzhikang1@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

include/trace/events/f2fs.h

@@ -149,6 +149,9 @@ TRACE_DEFINE_ENUM(CP_TRIMMED);
 		{ CP_SPEC_LOG_NUM,	"log type is 2" },		\
 		{ CP_RECOVER_DIR,	"dir needs recovery" })
 
+struct f2fs_sb_info;
+struct f2fs_io_info;
+struct extent_info;
 struct victim_sel_policy;
 struct f2fs_map_blocks;