parisc: regs_return_value() should return gpr28

While working on kretprobes for PA-RISC I was wondering while the
kprobes sanity test always fails on kretprobes. This is caused by
returning gpr20 instead of gpr28.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # 4.14+

arch/parisc/include/asm/ptrace.h

@@ -22,7 +22,7 @@ unsigned long profile_pc(struct pt_regs *);
 
 static inline unsigned long regs_return_value(struct pt_regs *regs)
 {
-	return regs->gr[20];
+	return regs->gr[28];
 }
 
 static inline void instruction_pointer_set(struct pt_regs *regs,