Update the TCP structure used by DTrace to show the smoothed RTT.
This will allow similar functionality to SIFTR to be built with DTrace.

Submitted by:	Grenville Armitage
MFC after:	2 weeks
WARNS=3 and style fixes.  No functionality change.
The U-Boot README says fdt_addr_r is the right env var for fdt data
loaded into ram, but vendors also use fdtaddr and fdt_addr.  Check the
recommended variable first and fall back to the others.