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.cn


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