perf record: Support synthesizing cgroup events

perf record: Support synthesizing cgroup events

Synthesize cgroup events by iterating cgroup filesystem directories.
The cgroup event only saves the portion of cgroup path after the mount
point and the cgroup id (which actually is a file handle).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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/20200325124536.2800725-7-namhyung@kernel.org
Link: http://lore.kernel.org/lkml/20200402015249.3800462-1-namhyung@kernel.org
[ Extracted the HAVE_FILE_HANDLE from the followup patch, added missing __maybe_unused ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>