libtracefs: Handle synthetic events with dynamic strings as fields

Dynamic strings may contain "__data_loc" in their types within the format
files, but "__data_loc" is not an acceptable type for the dynamic_events
file.

For example tracing the latency of interrupts and having the name of the
interrupt be reported:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  echo 's:irq_lat __data_loc char name[]; u64 delta;' >> /sys/kernel/tracing/dynamic_events
  echo 'hist:keys=irq:__arg_1193_2=name,__arg_1193_3=common_timestamp.usecs' >> /sys/kernel/tracing/events/irq/irq_handler_entry/trigger
  echo 'hist:keys=irq:__name_1193_1=$__arg_1193_2,__delta_1193_4=common_timestamp.usecs-$__arg_1193_3:onmatch(irq.irq_handler_entry).trace(irq_lat,$__name_1193_1,$__delta_1193_4)' >> /sys/kernel/tracing/events/irq/irq_handler_exit/trigger

The above writes echo 's:irq_lat __data_loc char name[]; u64 delta;' into
dynamic_events, and the "__data_loc" will cause it to fail. To show this,
adding the '-e' option will execute the command first:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  [23109.878734] synthetic_events: error: Invalid type
  Command: s:synthetic/irq_lat  __data_loc char name[]; u64 delta;
                                ^

The __data_loc caused the write to fail.

Strip the __data_loc off of fields so that it will produce:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  echo 's:irq_lat char name[]; u64 delta;' >> /sys/kernel/tracing/dynamic_events
  echo 'hist:keys=irq:__arg_1193_2=name,__arg_1193_3=common_timestamp.usecs' >> /sys/kernel/tracing/events/irq/irq_handler_entry/trigger
  echo 'hist:keys=irq:__name_1193_1=$__arg_1193_2,__delta_1193_4=common_timestamp.usecs-$__arg_1193_3:onmatch(irq.irq_handler_entry).trace(irq_lat,$__name_1193_1,$__delta_1193_4)' >> /sys/kernel/tracing/events/irq/irq_handler_exit/trigger

And not fail to execute.

Link: https://lore.kernel.org/20250410220439.14b81af3@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>