f2fs: init local extent_info to avoid stale stack info in tp

To avoid such stale(fops, blk, len) info in f2fs_lookup_extent_tree_end tp

dio-23095 [005] ...1 17878.856859: f2fs_lookup_extent_tree_end:
			dev = (259,30), ino = 856, pgofs = 0,
			ext_info(fofs: 3441207040, blk: 4294967232, len: 3481143808)

Signed-off-by: Hou Pengyang <houpengyang@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>