fgraph: Make overruns 4 bytes in graph stack structure

Inspecting the data structures of the function graph tracer, I found that
the overrun value is unsigned long, which is 8 bytes on a 64 bit machine,
and not only that, the depth is an int (4 bytes). The overrun can be simply
an unsigned int (4 bytes) and pack the ftrace_graph_ret structure better.

The depth is moved up next to the func, as it is used more often with func,
and improves cache locality.

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