perf ftrace: Fix latency stats with BPF

When BPF collects the stats for the latency in usec, it first divides
the time by 1000.  But that means it would have 0 if the delta is small
and won't update the total time properly.

Let's keep the stats in nsec always and adjust to usec before printing.

Before:

  $ sudo ./perf ftrace latency -ab -T mutex_lock --hide-empty -- sleep 0.1
  #   DURATION     |      COUNT | GRAPH                                          |
       0 -    1 us |        765 | #############################################  |
       1 -    2 us |         10 |                                                |
       2 -    4 us |          2 |                                                |
       4 -    8 us |          5 |                                                |

  # statistics  (in usec)
    total time:                    0    <<<--- (here)
      avg time:                    0
      max time:                    6
      min time:                    0
         count:                  782

After:

  $ sudo ./perf ftrace latency -ab -T mutex_lock --hide-empty -- sleep 0.1
  #   DURATION     |      COUNT | GRAPH                                          |
       0 -    1 us |        880 | ############################################   |
       1 -    2 us |         13 |                                                |
       2 -    4 us |          8 |                                                |
       4 -    8 us |          3 |                                                |

  # statistics  (in usec)
    total time:                  268    <<<--- (here)
      avg time:                    0
      max time:                    6
      min time:                    0
         count:                  904

Tested-by: Athira Rajeev <atrajeev@linux.ibm.com>
Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20250227191223.1288473-1-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>