selinux,smack: properly reference the LSM blob in security_watch_key()

Unfortunately when we migrated the lifecycle management of the key LSM
blob to the LSM framework we forgot to convert the security_watch_key()
callbacks for SELinux and Smack.  This patch corrects this by making use
of the selinux_key() and smack_key() helper functions respectively.

This patch also removes some input checking in the Smack callback as it
is no longer needed.

Fixes: 5f8d28f6 ("lsm: infrastructure management of the key security blob")
Reported-by:  <syzbot+044fdf24e96093584232@syzkaller.appspotmail.com>
Tested-by:  <syzbot+044fdf24e96093584232@syzkaller.appspotmail.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>