powerpc64/ftrace: use PACA to retrieve TOC in mod_return_to_handler

The mod_return_to_handler needs to switch to the kernel TOC before
jumping to a the kernel code. It currently does this by looking
at the kernel function data and retrieves the TOC that way.

Not only is this inefficient, it also breaks with a relocatable kernel.
The PACA contains the kernel TOC and we can easily retrieve it that
way.

Reported-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>