Adding control 'stop' command to stop perf record.

When it is received, perf will set the 'done' variable to 1 to stop its
mmap ring buffer reading loop.

Example session:

  terminal 1:
    # mkfifo control ack
    # perf record --control=fifo:control,ack

  terminal 2:
    # echo stop > control

  terminal 1:
    [ perf record: Woken up 7 times to write data ]
    [ perf record: Captured and wrote 3.214 MB perf.data (38280 samples) ]
    #
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Budankov <abudankov@huawei.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20201226232038.390883-4-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>