Changes

Refactor diffing same GMM messages

gprs_gmm_attach_req_ies() isn'nt specific for the attach request.
It also did not cover the full message because of a fixed message length
when comparing both messages.

A diff of the old and new GMM message is required to figure out
if the MS/UE is retransmitting an old message or starting
a new procedure.

Change-Id: Ie698d3a6894a5796663c22c8bfd12b47acda57e6