Introduce a new option to display events of type PERF_RECORD_NAMESPACES
and update perf-script documentation accordingly.

Shown below is output (trimmed) of perf script command with the newly
introduced option, on perf.data generated with perf record command using
--namespaces option.

  $ perf script --show-namespace-events
      swapper   0 [000]     0.000000: PERF_RECORD_NAMESPACES 1/1 - nr_namespaces: 7
                [0/net: 3/0xf000001c, 1/uts: 3/0xeffffffe, 2/ipc: 3/0xefffffff, 3/pid: 3/0xeffffffc,
                 4/user: 3/0xeffffffd, 5/mnt: 3/0xf0000000, 6/cgroup: 3/0xeffffffb]
      swapper   0 [000]     0.000000: PERF_RECORD_NAMESPACES 2/2 - nr_namespaces: 7
                [0/net: 3/0xf000001c, 1/uts: 3/0xeffffffe, 2/ipc: 3/0xefffffff, 3/pid: 3/0xeffffffc,
                 4/user: 3/0xeffffffd, 5/mnt: 3/0xf0000000, 6/cgroup: 3/0xeffffffb]

Commiter notes:

Testing it:

Investigating that double PERF_RECORD_NAMESPACES for the 19155
pid/tid... Its more than that, there are two PERF_RECORD_COMM as well,
and with zeroed timestamps, so probably a synthesizing artifact...

  # perf script --show-task --show-namespace
  <SNIP>
      perf     0 [000]     0.000000: PERF_RECORD_COMM: perf:19154/19154
      perf     0 [000]     0.000000: PERF_RECORD_FORK(19155:19155):(19154:19154)
      perf     0 [000]     0.000000: PERF_RECORD_NAMESPACES 19155/19155 - nr_namespaces: 7
          [0/net: 3/0xf0000081, 1/uts: 3/0xeffffffe, 2/ipc: 3/0xefffffff, 3/pid: 3/0xeffffffc,
           4/user: 3/0xeffffffd, 5/mnt: 3/0xf0000000, 6/cgroup: 3/0xeffffffb]
      perf     0 [000]     0.000000: PERF_RECORD_COMM: perf:19155/19155
      perf     0 [000]     0.000000: PERF_RECORD_COMM: perf:19155/19155
      perf     0 [000]     0.000000: PERF_RECORD_NAMESPACES 19155/19155 - nr_namespaces: 7
          [0/net: 3/0xf0000081, 1/uts: 3/0xeffffffe, 2/ipc: 3/0xefffffff, 3/pid: 3/0xeffffffc,
           4/user: 3/0xeffffffd, 5/mnt: 3/0xf0000000, 6/cgroup: 3/0xeffffffb]
   swapper     0 [000]  3110.881834:          1 cycles:  ffffffffa7060bf6 native_write_msr (/lib/modules/4.11.0-rc1+/build/vmlinux)

  <SNIP>
Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/148891932627.25309.1941587059154176221.stgit@hbathini.in.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>