SGSN: encrypt/decrypt only necessary frames

According to 3GPP TS 24.008 ยง 4.7.1.2 some GMM frames are not supposed
to be ciphered. Propagate information about the necessity for
encryption between MM <-> LLC to ensure only proper frames are
encrypted/decrypted/dropped.

Change-Id: I0358905e60d1b182f75caec81bfcc72bbbbb2aa1
Related: OS#1582
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index e8e05f6..1c95634 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -215,7 +215,7 @@
 	msg = create_msg(data, data_len);
 	msgb_tlli(msg) = tlli;
 	bssgp_create_cell_id(msgb_bcid(msg), bssgp_raid, 0);
-	gsm0408_gprs_rcvmsg_gb(msg, llme);
+	gsm0408_gprs_rcvmsg_gb(msg, llme, false);
 	msgb_free(msg);
 }