Changes

gtp: Rework gtp_resp() into gtp_resp_pdp()

gtp_resp() requires a PDP context. To support more generic messages
without a PDP context, rename gtp_resp() into gtp_resp_pdp() and
create a more generic function to transmit a GTP response.

Will be used by the SGSN Context Req/Resp/Ack code.

Change-Id: Id9ff95e0e2a10a22e65ecf42b2a2b06a0f2d1a45