Hide LRO code under #ifdef INET/INET6 to allow NOIP kernel to compile.

While the code comments indicate that LRO is currently only supported
for IPv4/TCP, we'll still compile it in for IPv6.

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271913 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f