perf bpf: Use ERR_CAST instead of ERR_PTR(PTR_ERR())

Use ERR_CAST inlined function instead of ERR_PTR(PTR_ERR(...)).  This
makes it more readable and also fix this warning detected by
err_cast.cocci:

  tools/perf/util/bpf-loader.c:1606:11-18: WARNING: ERR_CAST can be used with op
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wen Yang <yellowriver2010@hotmail.com>
Cc: zhong.weidong@zte.com.cn
Link: http://lkml.kernel.org/r/20181127090610.28488-1-wen.yang99@zte.com.cnSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf-loader.c

@@ -1603,7 +1603,7 @@ struct perf_evsel *bpf__setup_output_event(struct perf_evlist *evlist, const cha
 
 			op = bpf_map__add_newop(map, NULL);
 			if (IS_ERR(op))
-				return ERR_PTR(PTR_ERR(op));
+				return ERR_CAST(op);
 			op->op_type = BPF_MAP_OP_SET_EVSEL;
 			op->v.evsel = evsel;
 		}