Coverity complains that perf_marker is never unmapped.
Fix by unmapping it in perf_exit().

Fixes: Coverity CID 1507929
Fixes: 5584e2db ("tcg: add perfmap and jitdump")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230605114134.1169974-1-iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>