bssgp_bvc_fsm: Set/get maximum BSSGP PDU length

Add functions to get/set the maximum supported BSSGP PDU size by the NS
layer.

IPv4 and IPv6 should not matter since we can just enable IP
fragmentation and send NS PDUs up to 2**16 + bytes. Frame relay does not
support fragmentation and this is the reason we need to be aware of the
maximum PDU size. Luckily with 1600 bytes the MTU in frame relay can hold a
regular IP packet including NS/BSSGP overhead.

On the NS layer this corresponds to the size of an NS SDU in NS-UNITDATA
(3GPP TS 48.016 Ch. 9.2.10)

Change-Id: I9bb82ead27366b7370c9ff968e03ca2113ec11f0
Related: OS#4889
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 7da11cd..2327815 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -96,6 +96,8 @@
 bssgp_bvc_get_features_advertised;
 bssgp_bvc_get_features_received;
 bssgp_bvc_get_features_negotiated;
+bssgp_bvc_fsm_set_max_pdu_len;
+bssgp_bvc_fsm_get_max_pdu_len;
 
 osmo_fr_network_alloc;
 osmo_fr_link_alloc;