m68k: correct setting of struct user.u_ar0

Commit 6e16d89b ("Sanitize the type of
struct user.u_ar0") forgot to change the m68k setting code, causing the
following compiler warning:

    arch/m68k/kernel/process.c:338: warning: assignment makes integer from pointer without a cast
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/m68k/kernel/process.c

@@ -335,7 +335,7 @@ void dump_thread(struct pt_regs * regs, struct user * dump)
 	if (dump->start_stack < TASK_SIZE)
 		dump->u_ssize = ((unsigned long) (TASK_SIZE - dump->start_stack)) >> PAGE_SHIFT;
 
-	dump->u_ar0 = (struct user_regs_struct *)((int)&dump->regs - (int)dump);
+	dump->u_ar0 = offsetof(struct user, regs);
 	sw = ((struct switch_stack *)regs) - 1;
 	dump->regs.d1 = regs->d1;
 	dump->regs.d2 = regs->d2;