tracing: Use the return of str_has_prefix() to remove open coded numbers

There are several locations that compare constants to the beginning of
string variables to determine what commands should be done, then the
constant length is used to index into the string. This is error prone as the
hard coded numbers have to match the size of the constants. Instead, use the
len returned from str_has_prefix() and remove the open coded string length
sizes.

Cc: Joe Perches <joe@perches.com>
Acked-by: Masami  Hiramatsu <mhiramat@kernel.org> (for trace_probe part)
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>