kmod/core: ftrace function_graph tracer compatibility

Steven Rostedt recommended to return "regs->ip + MCOUNT_INSN_SIZE",
which is what the function_graph tracer expects.  This fixes
function_graph tracing for a patched function.

This change also means that the function tracer will only show the
patched function once (corresponding to a trace of the original
function) rather than twice.  This is probably more in line with what a
user would expect.