The GTP module is common for 4G and 5G. It hardcodes the functions to
call when creating S1U and NGU tunnels. The S1U case is both applicable
for LTE and NR (NSA), which until now was not a problem as the PDCP data
forwarding functions where called pdcp_data_req. The issue is that in this
branch, the NR PDCP functions have changed to nr_pdcp_data_req_drb().

When setting up a GTP tunnel, we now pass the PDCP data forwarding
functions into GTP, since the caller knows whether it is in 4G or 5G,
but GTP itself does not.