MIPS: Ftrace: Fix function tracing return address to match

Dynamic function tracing was not working on MIPS.  When doing dynamic
tracing, the tracer attempts to match up the passed in address with
the one the compiler creates in the mcount tables.  The MIPS code was
passing in the return address from the tracing function call, but the
compiler tables were the address of the function call.  So they
wouldn't match.

Just subtracting 8 from the return address will give the address of
the function call.  Easy enough.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
[david.daney@cavium.com: Adjusted code comment and patch Subject.]
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/5592/