fix hwpmc memory leaks

[markj]

Summary

hwpmc uses pmc_release_pmc_descriptor() to handle a bunch of PMC cleanup. This function also calls pmc_destroy_pmc_descriptor(), which currently doesn't do anything besides verify a few assertions.

In particular, pmc_release_pmc_descriptor() doesn't actually free() the descriptor - it's up to the caller to do that. There are a few places where this call was missing. I fixed this by changing pmc_destroy_pmc_descriptor() to free() the descriptor (this is consistent with pmc_destroy_owner_descriptor()), and adding missing calls.

Test Plan

It's possible to trigger a leak by running, say, "pmcstat -S unhalted-cycles -T" and killing the pmcstat process with SIGTERM. With this change, the memory leak is fixed and the command still works properly. Isilon's pmc tests pass with this change too.

[bz]

I think this looks ok. At least on first glance I didn't spot a problem or a double-free now.

[markj]

Closed by commit rS273236 (authored by @markj).