[SECURITY] secmark: nul-terminate secdata

The patch below fixes a problem in the iptables SECMARK target, where
the user-supplied 'selctx' string may not be nul-terminated.

From initial analysis, it seems that the strlen() called from
selinux_string_to_sid() could run until it arbitrarily finds a zero,
and possibly cause a kernel oops before then.

The impact of this appears limited because the operation requires
CAP_NET_ADMIN, which is essentially always root.  Also, the module is
not yet in wide use.

Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: David S. Miller <davem@davemloft.net>