Fix utils.FollowSymlinkInScope's infinite loops bug

fs_test.go doesn't finish if Docker's code is placed under a directory
which has symlinks between / and the directory.
For example, the below doesn't finish before the change.

  /home -> usr/home
  FollowSymlinkInScope("/home/bob/foo/bar", "/home/bob/foo")

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)