[rest_octets] Return bv.data_len to indicate how was written

Return the used data_len for the bitvector. This indicates
how much data might have been written.
diff --git a/openbsc/src/rest_octets.c b/openbsc/src/rest_octets.c
index 6c8c403..74874bd 100644
--- a/openbsc/src/rest_octets.c
+++ b/openbsc/src/rest_octets.c
@@ -46,7 +46,7 @@
 		bitvec_set_bit(&bv, L);
 
 	bitvec_spare_padding(&bv, 7);
-	return 0;
+	return bv.data_len;
 }
 
 /* Append selection parameters to bitvec */
@@ -125,7 +125,8 @@
 	/* GPRS Indicator */
 	append_gprs_ind(&bv, &si3->gprs_ind);
 
-	return bitvec_spare_padding(&bv, (bv.data_len*8)-1);
+	bitvec_spare_padding(&bv, (bv.data_len*8)-1);
+	return bv.data_len;
 }
 
 static int append_lsa_params(struct bitvec *bv,
@@ -178,7 +179,7 @@
 		bitvec_set_bit(&bv, si4->break_ind ? H : L);
 	}
 
-	return 0;
+	return bv.data_len;
 }
 
 /* GPRS Mobile Allocation as per TS 04.60 Chapter 12.10a:
@@ -390,5 +391,6 @@
 			}
 		}
 	}
-	return bitvec_spare_padding(&bv, (bv.data_len*8)-1);
+	bitvec_spare_padding(&bv, (bv.data_len*8)-1);
+	return bv.data_len;
 }