perf tools: Handle EINTR error for readn/writen

Those readn/writen functions are to ensure read/write does I/O for
a given size exactly.  But ion() - its implementation - does not
handle in case it returns prematurely due to a signal.  As it's not
an error itself so just retry the operation.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1398346054-3322-1-git-send-email-namhyung@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>