selinux: reject setexeccon() on MNT_NOSUID applications with -EACCES

We presently prevent processes from using setexecon() to set the
security label of exec()'d processes when NO_NEW_PRIVS is enabled by
returning an error; however, we silently ignore setexeccon() when
exec()'ing from a nosuid mounted filesystem.  This patch makes things
a bit more consistent by returning an error in the setexeccon()/nosuid
case.

Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>