perf evlist: Fix perf_evlist__mmap_read event overflow

perf evlist: Fix perf_evlist__mmap_read event overflow

The perf_evlist__mmap_read used 'union perf_event' as a placeholder for
event crossing the mmap boundary.

This is ok for sample shorter than ~PATH_MAX. However we could grow up
to the maximum sample size which is 16 bits max.

I hit this overflow issue when using 'perf top -G dwarf' which produces
sample with the size around 8192 bytes.  We could configure any valid
sample size here using: '-G dwarf,size'.

Using array with sample max size instead for the event placeholder. Also
adding another safe check for the dynamic size of the user stack.

TODO: The 'struct perf_mmap' is quite big now, maybe we could use some
lazy allocation for event_copy size.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: David Ahern <dsahern@gmail.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1380721599-24285-1-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>