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>

security/smack/smack_lsm.c

@@ -387,7 +387,7 @@ static int smack_sb_umount(struct vfsmount *mnt, int flags)
 	struct smk_audit_info ad;
 
 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_FS);
-	smk_ad_setfield_u_fs_path_dentry(&ad, mnt->mnt_mountpoint);
+	smk_ad_setfield_u_fs_path_dentry(&ad, mnt->mnt_root);
 	smk_ad_setfield_u_fs_path_mnt(&ad, mnt);
 
 	sbp = mnt->mnt_sb->s_security;