perf report: Add 'cgroup' sort key

perf report: Add 'cgroup' sort key

The cgroup sort key is to show cgroup membership of each task.
Currently it shows full path in the cgroupfs (not relative to the root
of cgroup namespace) since it'd be more intuitive IMHO.  Otherwise root
cgroup in different namespaces will all show same name - "/".

The cgroup sort key should come before cgroup_id otherwise
sort_dimension__add() will match it to cgroup_id as it only matches with
the given substring.

For example it will look like following.  Note that record patch adding
--all-cgroups patch will come later.

  $ perf record -a --namespace --all-cgroups  cgtest
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.208 MB perf.data (4090 samples) ]

  $ perf report -s cgroup_id,cgroup,pid
  ...
  # Overhead  cgroup id (dev/inode)  Cgroup          Pid:Command
  # ........  .....................  ..........  ...............
  #
      93.96%  0/0x0                  /                 0:swapper
       1.25%  3/0xeffffffb           /               278:looper0
       0.86%  3/0xf000015f           /sub/cgrp1      280:cgtest
       0.37%  3/0xf0000160           /sub/cgrp2      281:cgtest
       0.34%  3/0xf0000163           /sub/cgrp3      282:cgtest
       0.22%  3/0xeffffffb           /sub            278:looper0
       0.20%  3/0xeffffffb           /               280:cgtest
       0.15%  3/0xf0000163           /sub/cgrp3      285:looper3

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200325124536.2800725-6-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>