LDP: Fix a length check

LDP: Fix a length check

In ldp_tlv_print(), the FT Session TLV length must be 12, not 8 (RFC3479)

Moreover:
Update the TLV_TCHECK() macro to remove the useless ND_TCHECK_LEN() call
with the GET_ macros. Thus remain the length check giving a more accurate
output.

Rename a label.

Update the output of a test accordingly.

Add a comment.

Partial update from aa5c6b710dfd8020d2c908d6b3bd41f1da719b3b
in 4.9 branch.