mem-ruby: Fixed MOESI_CMP_directory resource tracking

mem-ruby: Fixed MOESI_CMP_directory resource tracking

Fixes a few resource allocation issues in the directory controller:

- Added TBE resource checks on allocation.
- Now also allocating a TBE when issuing read requests to the controller
  to allow for a better response to backpressure. Without the TBE as a
  limiting factor, the directory can have an unbounded amount of
  outstanding memory requests.
- Also allocating a TBE for forwarded requests.

Change-Id: I17016668bd64a50a4354baad5d181e6d3802ac46
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21928
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>