In ip_forward(), when calculating the MTU in effect for an IPSEC transport mode tunnel, take the per-route MTU into account, *if* and *only if* it is non-zero (as found in struct rt_metrics/rt_metrics_lite).
In ip_forward(), when calculating the MTU in effect for an IPSEC transport
mode tunnel, take the per-route MTU into account, *if* and *only if* it
is non-zero (as found in struct rt_metrics/rt_metrics_lite).

PR:		kern/42727
Obtained from:	NetBSD (ip_input.c rev 1.151)

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