tracing: Have zero size length in filter logic be full string

As strings in trace events may not have a nul terminating character, the
filter string compares use the defined string length for the field for the
compares.

The trace_marker records data slightly different than do normal events. It's
size is zero, meaning that the string is the rest of the array, and that the
string also ends with '\0'.

If the size is zero, assume that the string is nul terminated and read the
string in question as is.

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>