perf pmu: zfree() expects a pointer to a pointer to zero it after freeing its contents

[ Upstream commit 57f14b5a ]

An audit showed just this one problem with zfree(), fix it.

Fixes: 9fbc61f8

 ("perf pmu: Add support for PMU capabilities")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>