perf symbols: Check access permission when reading symbol files

There 2 problems when reading symbols files:

*  It doesn't report any errors even if when users specify symbol
   files which don't exist with --kallsyms or --vmlinux. The result
   just shows the address without symbols, which is not what is expected.
   So it's better to report errors and exit the program.

*  When using command perf report --kallsyms=/proc/kallsyms with a
   non-root user, symbols are resolved. Then select one symbol and
   annotate it, it reports the error as the following:
   Can't annotate __clear_user: No vmlinux file with build id xxx was
   found.

   The problem is caused by reading /proc/kcore without access permission.
   /proc/kcore requires CAP_SYS_RAWIO capability to access, so it needs to
   change access permission to allow a specific user to read /proc/kcore or
   use root to execute the perf command.

This patch is to report errors when symbol files specified by users
don't exist. And check access permission of /proc/kcore when reading it.

Signed-off-by: Li Zhang <zhlcindy@linux.vnet.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1434704253-2632-1-git-send-email-zhlcindy@linux.vnet.ibm.com


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