perf symbols: Clarify method to get DSO binary_type filename

perf symbols: Clarify method to get DSO binary_type filename

Using dso__binary_type_file() make it look like this function will
return a file, not just its filename, so rename it to:

	dso__read_binary_type_filename()

to make its purpose clear, just like we have:

	dso__read_running_kernel_build_id()

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-vkf3upzrfrxtr01wueej4xw4@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>