perf synthetic events: Avoid write of uninitialized memory when generating PERF_RECORD_MMAP* records

Account for alignment bytes in the zero-ing memset.

Fixes: 1a853e36 ("perf record: Allow specifying a pid to record")
Signed-off-by: NIan Rogers <irogers@google.com>
Acked-by: NJiri Olsa <jolsa@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210309234945.419254-1-irogers@google.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>