perf lock contention: Fix msan issue in lock_contention_read()

perf lock contention: Fix msan issue in lock_contention_read()

I got a report of a msan failure like below:

  $ sudo perf lock con -ab -- sleep 1
  ...
  ==224416==WARNING: MemorySanitizer: use-of-uninitialized-value
      #0 0x5651160d6c96 in lock_contention_read  util/bpf_lock_contention.c:290:8
      #1 0x565115f90870 in __cmd_contention  builtin-lock.c:1919:3
      #2 0x565115f90870 in cmd_lock  builtin-lock.c:2385:8
      #3 0x565115f03a83 in run_builtin  perf.c:330:11
      #4 0x565115f03756 in handle_internal_command  perf.c:384:8
      #5 0x565115f02d53 in run_argv  perf.c:428:2
      #6 0x565115f02d53 in main  perf.c:562:3
      #7 0x7f43553bc632 in __libc_start_main
      #8 0x565115e865a9 in _start

It was because the 'key' variable is not initialized.  Actually it'd be set
by bpf_map_get_next_key() but msan didn't seem to understand it.  Let's make
msan happy by initializing the variable.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230324001922.937634-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>