perf bench: Fix NULL check against wrong variable

perf bench: Fix NULL check against wrong variable

We did a NULL check after "epollfdp = calloc(...)", but we checked
"epollfd" instead of "epollfdp".

Signed-off-by: Weiguo Li <liwg06@foxmail.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/tencent_B5D64530EB9C7DBB8D2C88A0C790F1489D0A@qq.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>