GFS2: Fix refcnt leak on gfs2_follow_link() error path

If ->follow_link handler return the error, it should decrement
nd->path refcnt.

This patch fix it.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>