perf symbols: Introduce filename__readable to check readability

Introduce filename__readable to check readability by opening the file
directly. Since the access(R_OK) just checks the readability based on
real UID/GID, it is ignored that the effective UID/GID and capabilities
for some special file (e.g.  /proc/kcore).

filename__readable() directly opens given file with O_RDONLY so that the
kernel checks it by effective UID/GID and capabilities.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Hemant Kumar <hemant@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160528151513.16098.97576.stgit@devbox


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>