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>

arch/powerpc/kernel/entry_64.S

@@ -1038,8 +1038,7 @@ _GLOBAL(mod_return_to_handler)
 	 * We are in a module using the module's TOC.
 	 * Switch to our TOC to run inside the core kernel.
 	 */
-	LOAD_REG_IMMEDIATE(r4,ftrace_return_to_handler)
-	ld	r2, 8(r4)
+	ld	r2, PACATOC(r13)
 
 	bl	.ftrace_return_to_handler
 	nop