netinet6: Remove set but not used global variable in6_maxmtu

[zlei]

Summary

This was introduced by the KAME project, but it appears to serve no purpose.

[glebius]

sys/netinet6/nd6_rtr.c

555–560

This can be simplified and avoids extra memory write if MTU didn't change.

		if (mtu <= maxmtu && ndi->linkmtu != mtu);
			ndi->linkmtu = mtu;
			rt_updatemtu(ifp);
		} else {

[zlei]

sys/netinet6/nd6_rtr.c

555–560

This can be simplified and avoids extra memory write if MTU didn't change.

Emm, then the else block

	nd6log((LOG_INFO, "%s: bogus mtu=%lu sent from %

may lost reporting when mtu > maxmtu && ndi->linkmtu == mtu .

[zlei]

sys/netinet6/nd6_rtr.c

555–560

may lost reporting when mtu > maxmtu && ndi->linkmtu == mtu

The above is wrong.

May falsely report bogus mtu when ndi->linkmtu == mtu.

[zlei]

Removed local var change.