Fix bounce

[vext01]

We've been experiencing non-deterministic segfaults, assertion failures
and Lua-level errors in bounce.lua for a while:
#1544

After a couple of days of systematically narrowing down the cause, it
appears to be something to do with the re-use of the PT resources in
collect.c.

In theory reusing the PT stuff for subsequent tracing sessions on the
same thread should be just fine -- I've even found examples on the
internet where people do this -- however, something is wrong with our
implementation, so I'm disabling the caching for now.

This commit does several things related to this:

• Fixes a bug in the aux buffer read code in the case that the ring
  buffer wraps around. We were incorrectly bumping the length of the
  recorded trace, which would cause trace corruption in the form of
  uninitialised reads when we come to decode the trace:

  -    trace->len += size + head;
  +    trace->len += head;

  Later changes (not re-using the aux buffer) will actually mean that
  the wraparound case can't happen (since the aux buffer is the same
  size as the allocated copyout buffer), but I'm leaving it in in case
  we ever decide to re-use perf stuff again.

  Sadly this fix alone isn't enough to fix bounce.lua.

• Removes caching of perf-related mmap buffers and the perf file
  descriptor itself. This basically reverts bedafd9. This means each
  tracing session now allocates fresh memory and opens a fresh perf fd.
  As part of this change we re-introduce PacketParserState::Init and
  add a couple of other permissible packets that I've seen come up when
  using try_repeat on the test suite.

• Ensures that the collector's resources are freed by calling
  hwt_perf_free_collector(), even if collecting a trace is aborted by
  (e.g.) "trace too long". We were failing to do this, meaning that
  resources would potentially leak. Since we no longer cache the perf
  fd, and this is closed in hwt_perf_free_collector(), if we don't
  call that, then the thread is effectively blocked from ever tracing
  again: trying to open perf agaiin would give "resource busy".

This appears to fix the original bounce.lua and the reduced one that me an Lukas devised.

I did 3 try_ci runs of this and all suceeded! Fingers crossed.

[vext01]

gah.

[ltratt]

Please force push an update.

[vext01]

Force pushed the fix. Sorry about that!