isisd: ignore the unrecognized TLVs

When processing LSPDUs, the unrecognized TLVs/sub-TLVs should be
silently ignored.

In parse_tlvs(), ISIS_WARNING is returned once an unrecognized TLV
exists. It breaks the processing in lsp_authentication_check() and
lsp_update_data(). So remove it.

Signed-off-by: Feng Lu <lu.feng@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/isisd/isis_tlv.c b/isisd/isis_tlv.c
index 2c2415a..bbfa5d8 100644
--- a/isisd/isis_tlv.c
+++ b/isisd/isis_tlv.c
@@ -742,7 +742,6 @@
 	  zlog_warn ("ISIS-TLV (%s): unsupported TLV type %d, length %d",
 		     areatag, type, length);
 
-	  retval = ISIS_WARNING;
 	  pnt += length;
 	  break;
 	}