add some comments regarding the free() / release of pdp contextx

It might be that we're deleting structures instead of properly cleaning
up the attached libgtp handles...
diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index 0ed7162..72d9e76 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -604,6 +604,9 @@
 					ictx->p_tmsi, ctx->p_tmsi);
 				gprs_llgmm_assign(ictx->llme, ictx->tlli,
 						  0xffffffff, GPRS_ALGO_GEA0, NULL);
+				/* FIXME: this is a hard free, we don't
+				 * clean-up any PDP contexts on the
+				 * libgtp side */
 				sgsn_mm_ctx_free(ictx);
 			}
 		}