selftest: ftrace: Fix to add 256 kprobe events correctly

Current multiple-kprobe testcase only tries to add
kprobe events on first 256 text symbols. However
kprobes fails to probe on some text symbols (like
blacklisted symbols). Thus in the worst case,
the test can not add any kprobe events.

To avoid that, continue to try adding kprobe events
until 256 events. Also it confirms the number of
registered kprobe events.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>