selinux: Print 'sclass' as string when unrecognized netlink message occurs

This prints the 'sclass' field as string instead of index in unrecognized netlink message.
The textual representation makes it easier to distinguish the right class.

Signed-off-by: Marek Milkovic <mmilkovi@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: 80-char width fixes]
Signed-off-by: Paul Moore <pmoore@redhat.com>