tracing: Add snapshot trigger to function probes

 echo 'schedule:snapshot:1' > /debug/tracing/set_ftrace_filter

This will cause the scheduler to trigger a snapshot the next time
it's called (you can use any function that's not called by NMI).

Even though it triggers only once, you still need to remove it with:

 echo '!schedule:snapshot:0' > /debug/tracing/set_ftrace_filter

The :1 can be left off for the first command:

 echo 'schedule:snapshot' > /debug/tracing/set_ftrace_filter

But this will cause all calls to schedule to trigger a snapshot.
This must be removed without the ':0'

 echo '!schedule:snapshot' > /debug/tracing/set_ftrace_filter

As adding a "count" is a different operation (internally).

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>