Changes

gtp: split gtp_req into 2 parts: transmit and fill header

Split the preparing the data from the socket and queue handling into two functions:
gtp_req(): preparing the data
gtp_req_transmit(): selecting socket, port and adding it to the transmit queue.
In a following commit gtp_req_transmit() can be used by gtpv1 function
as well.

Change-Id: Icda0ef7b0ce3631e23da88827dd54cf019878b5d