perf jevents: Fix suspicious code in fixregex()

perf jevents: Fix suspicious code in fixregex()

The new string should have enough space for the original string and the
back slashes IMHO.

Fixes: fbc2844e84038ce3 ("perf vendor events: Use more flexible pattern matching for CPU identification for mapfile.csv")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: William Cohen <wcohen@redhat.com>
Link: http://lore.kernel.org/lkml/20200903152510.489233-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>