tracing: Fix regex_match_front() to not over compare the test string

The regex match function regex_match_front() in the tracing filter logic,
was fixed to test just the pattern length from testing the entire test
string. That is, it went from strncmp(str, r->pattern, len) to
strcmp(str, r->pattern, r->len).

The issue is that str is not guaranteed to be nul terminated, and if r->len
is greater than the length of str, it can access more memory than is
allocated.

The solution is to add a simple test if (len < r->len) return 0.

Cc: stable@vger.kernel.org
Fixes: 285caad4

 ("tracing/filters: Fix MATCH_FRONT_ONLY filter matching")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>