perf symbols: Read plt symbols from proper symtab_type binary

When loading symbols from DSO we check multiple paths of DSO binary
until we succeed to load symbols ('.symtab' section). Once symbols are
read we try to load also plt symbols.

During the reading of plt symbols, the dso file is reopened from
location given by dso->long_name. This could be wrong in case we want
process buildid binaries.

The change is to make the plt symbols being read from the DSO path, that
normal symbols were read from.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1334756818-6631-1-git-send-email-jolsa@redhat.com
[ committer note: moved dso to be the first parameter of that function ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>