perf tools: Update copy of libbpf's hashmap.c

perf tools: Update copy of libbpf's hashmap.c

To pick the changes in:

  fba60b171a032283 ("libbpf: Use IS_ERR_OR_NULL() in hashmap__free()")

That don't entail any changes in tools/perf.

This addresses this perf build warning:

  Warning: Kernel ABI header at 'tools/perf/util/hashmap.h' differs from latest version at 'tools/lib/bpf/hashmap.h'
  diff -u tools/perf/util/hashmap.h tools/lib/bpf/hashmap.h

Not a kernel ABI, its just that this uses the mechanism in place for
checking kernel ABI files drift.

Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mauricio Vásquez <mauricio@kinvolk.io>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/YkMb2SAIai2VeuUD@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>