Smack: fix another vsscanf out of bounds

This is similar to commit 84e99e58 ("Smack: slab-out-of-bounds in
vsscanf") where we added a bounds check on "rule".

Reported-by:  <syzbot+a22c6092d003d6fe1122@syzkaller.appspotmail.com>
Fixes: f7112e6c ("Smack: allow for significantly longer Smack labels v4")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>