isisd: Add SRv6 MSDs to Router Capabilities TLV

Add support for SRv6 Node MSDs as per RFC 9352 section #4.

There are four types of SRv6 MSDs:
* Maximum Segments Left MSD Type
* Maximum End Pop MSD Type
* Maximum H.Encaps MSD Type
* Maximum End D MSD Type

These SRv6 Node MSDs are advertised in the Node MSD Sub-TLV, a Sub-TLV
of the Router Capabilities TLV.

This commit adds `struct isis_srv6_msd` data structure, which represents
the SRv6 MSDs advertised in the Node Sub-TLV.

This commit also adds the `struct isis_srv6_msd` to
`struct isis_router_cap`, a data structure representing the Router
Capabilities TLV.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>

isisd/isis_tlvs.h

@@ -193,6 +193,18 @@ struct isis_router_cap_fad {
 };
 #endif /* ifndef FABRICD */
 
+/* Maximum SRv6 SID Depths (MSD) as per RFC 9352 section #4 */
+struct isis_srv6_msd {
+	/* RFC 9352 section #4.1 */
+	uint8_t max_seg_left_msd;
+	/* RFC 9352 section #4.2 */
+	uint8_t max_end_pop_msd;
+	/* RFC 9352 section #4.3 */
+	uint8_t max_h_encaps_msd;
+	/* RFC 9352 section #4.4 */
+	uint8_t max_end_d_msd;
+};
+
 /* SRv6 Capabilities as per RFC 9352 section #2 */
 struct isis_srv6_cap {
 	bool is_srv6_capable;
@@ -221,6 +233,9 @@ struct isis_router_cap {
 
 	/* RFC 9352 section #2 */
 	struct isis_srv6_cap srv6_cap;
+
+	/* RFC 9352 section #4 */
+	struct isis_srv6_msd srv6_msd;
 };
 
 struct isis_item {