tracing: Do not warn when connecting eprobe to non existing event

When the syscall trace points are not configured in, the kselftests for
ftrace will try to attach an event probe (eprobe) to one of the system
call trace points. This triggered a WARNING, because the failure only
expects to see memory issues. But this is not the only failure. The user
may attempt to attach to a non existent event, and the kernel must not
warn about it.

Link: https://lkml.kernel.org/r/20211027120854.0680aa0f@gandalf.local.home

Fixes: 7491e2c4

 ("tracing: Add a probe that attaches to trace events")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>