gprs_gmm: gsm48_rx_gmm_att_req(): drop stale PDP contexts on re-Attach
When an MS sends an Attach Request with a known IMSI or P-TMSI, it implies the MS has restarted and lost all its PDP context state. The SGSN must clean up any PDP contexts it still holds for that subscriber; otherwise it will not create new ones upon subsequent PDP Context Activation, causing data connectivity failure after a device reboot.
