sgsn: Make P-TMSI a local TLLI to avoid clash

Some broken equipment does not convert the P-TMSI to a Local TLLI.
This leads to the SGSN ignoring the GPRS Attach Complete message
from the phone. Proprietary SGSNs and some documentation we found
state that one should always set the two highest bits of a P-TMSI
to one. This will help broken equipment and will avoid a potential
P-TMSI/TLLI clash. The P-TMSI/Local TLLI mapping is now bijective.

openbsc/src/gprs/gprs_sgsn.c

@@ -361,7 +361,7 @@ uint32_t sgsn_alloc_ptmsi(void)
 	uint32_t ptmsi;
 
 restart:
-	ptmsi = rand();
+	ptmsi = rand() | 0xC0000000;
 	llist_for_each_entry(mm, &sgsn_mm_ctxts, list) {
 		if (mm->p_tmsi == ptmsi)
 			goto restart;