f2fs: fix wrong error hanlder in f2fs_follow_link

The page_follow_link_light returns NULL and its error pointer was remained
in nd->path.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>