sh: Fix the value of MCOUNT_INSN_OFFSET

It seems that MCOUNT_INSN_OFFSET was calculating the distance between
the wrong functions. The value that should have actually been computed
is the distance between ftrace_call and ftrace_stub. I discovered this
when I added some code to ftrace_caller.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>