trace: rename trace_enum_mutex to trace_eval_mutex

There is a lock protecting the trace_enum_map, rename
it to reflect the use by more than enums.

Link: http://lkml.kernel.org/r/20170531215653.3240-6-jeremy.linton@arm.com



Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>