The current code will always use the current stacktrace as a key even
if a stacktrace contained in a specific event field was specified.

For example, we expect to use the 'unsigned long[] stack' field in the
below event in the histogram:

  # echo 's:block_lat pid_t pid; u64 delta; unsigned long[] stack;' > /sys/kernel/debug/tracing/dynamic_events
  # echo 'hist:keys=delta.buckets=100,stack.stacktrace:sort=delta' > /sys/kernel/debug/tracing/events/synthetic/block_lat/trigger

But in fact, when we type out the trigger, we see that it's using the
plain old global 'stacktrace' as the key, which is just the stacktrace
when the event was hit and not the stacktrace contained in the event,
which is what we want:

  # 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]

And in fact, there's no code to actually retrieve it from the event,
so we need to add HIST_FIELD_FN_STACK and hist_field_stack() to get it
and hook it into the trigger code.  For now, since the stack is just
using dynamic strings, this could just use the dynamic string
function, but it seems cleaner to have a dedicated function an be able
to tweak independently as necessary.

Link: https://lkml.kernel.org/r/11aa614c82976adbfa4ea763dbe885b5fb01d59c.1676063532.git.zanussi@kernel.orgSigned-off-by: Tom Zanussi <zanussi@kernel.org>
[ Fixed 32bit build warning reported by kernel test robot <lkp@intel.com> ]
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>