ospf: clean up some length checks.

ospf: clean up some length checks.

In the loop for LS_OPAQUE_TE_TLV_LINK, loop until the remaining TLV
length is 0; we're already checking, first thing in the loop, that the
remaining length is at least 4, so we can process the T and the L.

For each sub-TLV of LS_OPAQUE_TE_TLV_LINK, make sure the sub-tlV doesn't
go past the end of the TLV.

If we compute the padded TLV or sub-TLV length, redo the check, to make
sure the padded length isn't too big.