TLToAXI4: block TL early source re-use before it goes to AXI4 (#1110)

This is a follow-up to PR #1108.

Rather than increasing the number of transactions we allow to be inflight,
instead just block TL when early source re-use happens. This is a better
fix since it means we don't pay mostly wasted downstream hardware to handle
an additional transaction inflight that almost never happens.