linux/perf.c: fix --linux_perf_bts_block startup case
linux/perf.c: fix --linux_perf_bts_block startup case

The failure:
$ honggfuzz -f in/ --linux_perf_bts_block -- /usr/bin/tiff -D ___FILE___
  [2016-11-11T23:11:20+0000][W][5951] arch_perfOpen():223 mmap(mmapAuxBuf) failed,
    try increasing the kernel.perf_event_mlock_kb sysctl (up to even 300000000): Cannot allocate memory
  [2016-11-11T23:11:20+0000][E][5951] arch_perfEnable():261 Cannot set up perf for PID=5953 (_HF_DYNFILE_BTS_BLOCK)
  [2016-11-11T23:11:20+0000][F][5951] arch_prepareChild():248 Couldn't enable perf counters for pid 5953

It seems aux data also needs WRITE permissions.

At least this change fixes startup for me.

Reported-by: Mateusz Lenik
Signed-off-by: Sergei Trofimovich <df7d2cee2e7ececeb2368216a0f9d0537fd32149@google.com>

anestisb/honggfuzz,google/honggfuzz,google/honggfuzz,google/honggfuzz