tracing: Save the last hist command's associated event name

tracing: Save the last hist command's associated event name

In preparation for making use of the new trace error log, save the
subsystem and event name associated with the last hist command - it
will be passed as the location param in the event_log_err() calls.

Link: http://lkml.kernel.org/r/eb0fd1362be8f39facb86c83eecf441b7a5876f8.1554072478.git.tom.zanussi@linux.intel.com
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>