libperf tests: Avoid uninitialized variable warning

The variable 'bf' is read (for a write call) without being initialized
triggering a memory sanitizer warning. Use 'bf' in the read and switch
the write to reading from a string.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210114212304.4018119-1-irogers@google.com


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