[PARISC] fix null ptr deref in unwind.c

commit ffb45122 removed one too many args.
kallsyms_lookup is not safe to call with a NULL *modname. Paper bag over the
problem for the time being.
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

arch/parisc/kernel/unwind.c

@@ -216,8 +216,10 @@ static void unwind_frame_regs(struct unwind_frame_info *info)
 		/* Handle some frequent special cases.... */
 		{
 			char symname[KSYM_NAME_LEN+1];
+			char *modname;
 
-			kallsyms_lookup(info->ip, NULL, NULL, NULL, symname);
+			kallsyms_lookup(info->ip, NULL, NULL, &modname,
+				symname);
 
 			dbg("info->ip = 0x%lx, name = %s\n", info->ip, symname);