perf ftrace: Add option '-m/--buffer-size' to set per-cpu buffer size

perf ftrace: Add option '-m/--buffer-size' to set per-cpu buffer size

This adds an option '-m/--buffer-size' to allow us set the size of per-cpu
tracing buffer.

Committer testing:

Before running with this option:

  # find /sys/kernel/tracing/ -name buffer_size_kb | xargs cat
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  1408
  #

Then, run:

  # perf ftrace -m 2048K | head -10
   2)               |  mutex_unlock() {
   2)   ==========> |
   2)               |    smp_irq_work_interrupt() {
   2)               |      irq_enter() {
   2)   0.121 us    |        rcu_irq_enter();
   2)   0.128 us    |        irqtime_account_irq();
   2)   0.719 us    |      }
   2)               |      __wake_up() {
   2)               |        __wake_up_common_lock() {
   2)   0.105 us    |          _raw_spin_lock_irqsave();
  #

Now look at those tracefs knobs:

  # find /sys/kernel/tracing/ -name buffer_size_kb | xargs cat
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  2048
  #

This should be similar to the -m option in the other perf tools, such as
'perf record', 'perf trace', etc.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-5-changbin.du@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>