selinux: cleanup error reporting in selinux_nlmsg_perm()

commit e173fb26 upstream.

Convert audit_log() call to WARN_ONCE().

Rename "type=" to nlmsg_type=" to avoid confusion with the audit record
type.

Added "protocol=" to help track down which protocol (NETLINK_AUDIT?) was used
within the netlink protocol family.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[Rewrote the patch subject line]
Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>