kcsan: Fix function matching in report

Pass string length as returned by scnprintf() to strnstr(), since
strnstr() searches exactly len bytes in haystack, even if it contains a
NUL-terminator before haystack+len.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>