XDE should listen to MSS suggestions made by senders for rack-external traffic

[FelixMcFelix]

The sender (illumos, viona) may have learned and deliberately set an MSS value on LSO traffic lower than the standard Ethernet MTU, accounting for tunnels and the like.

opte/xde/src/xde.rs

Lines 1779 to 1788 in c9c3d51

	// Boost MSS to use full jumbo frames if we know our path
	// can be served purely on internal links.
	// Recall that SDU does not include L2 size, hence 'non_eth_payl'
	let mut flags = offload_flags;
	let inner_mtu = if mtu_unrestricted {
	src_dev.underlay_capab.mtu - encap_len
	} else {
	u32::from(ETHERNET_MTU)
	};
	let mss = inner_mtu - non_eth_payl_bytes;

When we don't see mtu_unrestricted, we should be falling back to the value included in the dblk_t.

Part of unblocking PMTUD as in oxidecomputer/omicron#7998.