kprobes/ftrace: Recover original IP if pre_handler doesn't change it

Recover original IP register if the pre_handler doesn't change it.
Since current kprobes doesn't expect that another ftrace handler
may change regs->ip, it sets kprobe.addr + MCOUNT_INSN_SIZE to
regs->ip and returns to ftrace.
This seems wrong behavior since kprobes can recover regs->ip
and safely pass it to another handler.

This adds code which recovers original regs->ip passed from
ftrace right before returning to ftrace, so that another ftrace
user can change regs->ip.

Link: http://lkml.kernel.org/r/20141009130106.4698.26362.stgit@kbuild-f20.novalocal

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>