gprs_gmm.c: Send XID reset with received TLLI

Otherwise lower layers will end up using a TLLI from PTMSI which was not
yet announced to the MS if it is still not in GMM attached state, as
showcased by SGSN_Tests.TC_attach_req_id_req_ra_update.

Related: OS#3957, OS#4245
Change-Id: Ide51726abb82f5784eca4ab8d62b2ad8512be843