perf namespaces: Add reference count checking

Add reference count checking controlled by REFCNT_CHECKING ifdef. The
reference count checking interposes an allocated pointer between the
reference counted struct on a get and frees the pointer on a put.
Accesses after a put cause faults and use after free, missed puts are
caughts as leaks and double puts are double frees.

This checking helped resolve a memory leak and use after free:
https://lore.kernel.org/linux-perf-users/CAP-5=fWZH20L4kv-BwVtGLwR=Em3AOOT+Q4QGivvQuYn5AsPRg@mail.gmail.com/

Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexey Bayduraev <[email protected]>
Cc: Dmitriy Vyukov <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Riccardo Mancini <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Stephen Brennan <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
[ Extracted from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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