The defines in mbuf.h lro_tcp_d_len, lro_tcp_d_csum, and lro_tcp_h_off
are private to tcp_lro.c and lro_etype is tied to
M_LRO_EHDRSTRP (M_PROTO6) already defined in tcp_lro.h.
They are all "Layer specific non-persistent local storage" as mbuf.h
calls the fields, so move them over to tcp_lro.h where they belong.
MFC after: 8 weeks