fix mnt_mountpoint abuse in smack

(mnt,mnt_mountpoint) pair is conceptually wrong; if you want
to use it for generating pathname and for nothing else *and*
if you know that vfsmount tree is unchanging, you can get
away with that, but the right solution for that is (mnt,mnt_root).

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