sysprof: kernel trace

add kernel backtracing to the sysprof tracer.

change the format of the data, so that type=0 means
beginning of stack trace, 1 means kernel address, 2 means user
address, and 3 means end of trace.

EIP addresses are no longer distinguished from return addresses,
mostly because sysprof userspace doesn't make use of it. It may be
worthwhile adding this back in though, just in case it becomes
interesting.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>