tcp: Avoid conditionally defined fields in union lro_address

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.

As a side effect, this eliminates some duplication in the kernel's CTF
type graph.

Reviewed by: rscheff, tuexen
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34242