isisd: fix erroneous srv6 information in database

isisd: fix erroneous srv6 information in database

The show isis database detail command dumps invalid srv6 information:
>  SRv6 Locator: fc00:0:6::/64 (Metric: 0) ipv6-unicast
>    Sub-TLVs:
>      SRv6 End SID Endpoint Behavior: unknown, SID value: fc00:0:6:0:1::
>
>  MT Reachability: 0123.6452.1973.03 (Metric: 10) ipv6-unicast
>    Local Interface IPv6 Address(es): 192::4:3
>    SRv6 Lan End.X SID: fc00:0:3:0:43::, Algorithm: SPF, Weight: 0, Endpoint Behavior: End.DX6, Flags: B:0, S:0, P:0 Neighbor-ID: 0123.6452.1975
>        SRv6 SID Structure Locator Block length: 40, Locator Node length: 24, Function length: 16, Argument length: 0,

The behavior codepoint should use the IANA definitions to display the
correct value. Fix this by calling the appropriate convert function.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>