This reverts commit b6ab076d6b.
It turns out setting USE_MEMFD to 0 is really bad because it means we can't
resize the pool. Besides, valgrind SVN handles memfd so we really don't
need this fallback for valgrind anymore.
We leave the block pool untracked so that reads/writes to freed blocks
will get caught and do the tracking at the state pool/stream level. We
have to do a few extra gymnastics for streams because valgrind works in
terms of poitners and we work in terms of separate map and offset.
Fortunately, the users of the state pool and stream should always be using
the map pointer provided in the anv_state structure. We just have to
track, per block, the map that was used when we initially got the block.
Then we can make sure we always use that map and valgrind should stay
happy.
