selinux: fix residual uses of current_security() for the SELinux blob

We need to use selinux_cred() to fetch the SELinux cred blob instead
of directly using current->security or current_security().  There
were a couple of lingering uses of current_security() in the SELinux code
that were apparently missed during the earlier conversions. IIUC, this
would only manifest as a bug if multiple security modules including
SELinux are enabled and SELinux is not first in the lsm order. After
this change, there appear to be no other users of current_security()
in-tree; perhaps we should remove it altogether.

Fixes: bbd3662a ("Infrastructure management of the cred security blob")
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>