autofs: use IS_ROOT to replace root dentry checks

Use the helper macro !IS_ROOT to replace parent != dentry->d_parent.  Just
clean up.

Signed-off-by: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>