autofs: get rid of pointless checks around ->count handling

* IS_ROOT can't be true for unlink or rmdir victim
* any positive autofs dentry has non-NULL autofs_dentry_ino()
* autofs symlink can't have ->count other than 1
* autofs empty directory can't have ->count other than 1

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>