To help us provide a simple setns() in older distros.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-o10a85kf6j7ig87ep6crab2k@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>