net: tcp: move GRO/GSO functions to tcp_offload

net: tcp: move GRO/GSO functions to tcp_offload

Would be good to make things explicit and move those functions to
a new file called tcp_offload.c, thus make this similar to tcpv6_offload.c.
While moving all related functions into tcp_offload.c, we can also
make some of them static, since they are only used there. Also, add
an explicit registration function.

Suggested-by: Eric Dumazet <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>