perf symbols: debuglink should take symfs option into account

Currently code that tries to read corresponding debug symbol file from
.gnu_debuglink section (DSO_BINARY_TYPE__DEBUGLINK) does not take in
account symfs option, so filename__read_debuglink function cannot open
ELF file, if symfs option is used.

Fix is to add proper handling of symfs as it is done in other places:
use __symbol__join_symfs function to get real file name of target ELF
file.

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Tested-by: David Ahern <dsahern@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Anton Blanchard <anton@samba.org>
Cc: Avi Kivity <avi@cloudius-systems.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1422340442-4673-3-git-send-email-victor.kamensky@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>