Fix "tunnels: Don't apply GRO to multiple layers of encapsulation."

Commit dd4fff23 which is commit
fac8e0f579695a3ecbc4d3cac369139d7f819971 upstream, seems to have
included the sit_gro_receive function, yet it never is used, causing an
obvious warning message.  Hook it up to the correct sit_offload
structure.

Note, for 3.16, the backport of fac8e0f579695a3ecbc4d3cac369139d7f819971
does not include this function, nor the ipip case.  I'm guessing that
this is not correct for 3.18, as one of the functions was included, but
could be totally wrong.

Cc: Jesse Gross <jesse@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>