perf tools: Retry mapping buffers readonly on EACCES

Persistent event buffers may only be mmapped readonly. Thus, retry
mapping it readonly if mmap returns EACCES after trying to mmap
writable.

[ namhyung: Don't write to readonly mmap'ed buffers. ]
Signed-off-by: Robert Richter <robert.richter@linaro.org>
Signed-off-by: Robert Richter <rric@kernel.org>