tcp: Avoid conditionally defined fields in union lro_address

[markj]

Summary

The layout of the structure ends up depending on whether the including
file includes opt_inet.h and opt_inet6.h, so different compilation units
can end up seeing different versions of the structure. Fix this by
unconditionally defining the address fields.

[markj]

I found this because there are multiple definitions of lro_address, lro_parser, etc. in the CTF info for the kernel, caused by this mismatch. In general it's a bad idea to conditionally define fields in header files, unless it is guaranteed that all compilation units will evaluate the condition the same way.

[rscheff]