perf bench: The do_run_multi_threaded() function must use IS_ERR(perf_session__new())

In case of error, the function perf_session__new() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR()

Committer notes:

This wasn't compiling due to an extraneous '{' not matched by a '}', fix
it.

Fixes: 13edc237 ("perf bench: Add a multi-threaded synthesize benchmark")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200902140526.26916-1-yuehaibing@huawei.com


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