tracing/histogram: Fix a few problems with stacktrace variable printing

tracing/histogram: Fix a few problems with stacktrace variable printing

Currently, there are a few problems when printing hist triggers and
trace output when using stacktrace variables.  This fixes the problems
seen below:

  # echo 'hist:keys=delta.buckets=100,stack.stacktrace:sort=delta' > /sys/kernel/debug/tracing/events/synthetic/block_lat/trigger
  # cat /sys/kernel/debug/tracing/events/synthetic/block_lat/trigger
  hist:keys=delta.buckets=100,stacktrace:vals=hitcount:sort=delta.buckets=100:size=2048 [active]

  # echo 'hist:keys=next_pid:ts=common_timestamp.usecs,st=stacktrace  if prev_state == 2' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
  # cat /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
  hist:keys=next_pid:vals=hitcount:ts=common_timestamp.usecs,st=stacktrace.stacktrace:sort=hitcount:size=2048:clock=global if prev_state == 2 [active]

and also in the trace output (should be stack.stacktrace):

  {  delta: ~ 100-199, stacktrace         __schedule+0xa19/0x1520

Link: https://lkml.kernel.org/r/60bebd4e546728e012a7a2bcbf58716d48ba6edb.1676063532.git.zanussi@kernel.org
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>