Unabuse struct tcb.auxstr

As we've got a proper mechanism that parsers can use for storing private
data between entering and exiting stages, all cases of struct tcb.auxstr
abuse should go.

* btrfs.c (btrfs_ioctl): Use set_tcb_priv_ulong and get_tcb_priv_ulong
instead of abusing tcp->auxstr.
* file_handle.c (SYS_FUNC(name_to_handle_at)): Likewise.
* msghdr.c (SYS_FUNC(recvmsg)): Likewise.
* net.c (decode_sockname, SYS_FUNC(recvfrom)): Likewise.
* v4l2.c (print_v4l2_queryctrl): Likewise.
diff --git a/v4l2.c b/v4l2.c
index e80de55..237766e 100644
--- a/v4l2.c
+++ b/v4l2.c
@@ -571,14 +571,15 @@
 			tprints("}");
 			return 1;
 		}
-		if (tcp->auxstr)
+		if (get_tcb_priv_ulong(tcp))
 			tprints(" => ");
 	}
 
-	if (entering(tcp) || tcp->auxstr) {
+	if (entering(tcp) || get_tcb_priv_ulong(tcp)) {
 #ifdef V4L2_CTRL_FLAG_NEXT_CTRL
-		tcp->auxstr = (c.id & V4L2_CTRL_FLAG_NEXT_CTRL) ? "" : NULL;
-		if (tcp->auxstr) {
+		const unsigned long next = c.id & V4L2_CTRL_FLAG_NEXT_CTRL;
+		set_tcb_priv_ulong(tcp, next);
+		if (next) {
 			tprints("V4L2_CTRL_FLAG_NEXT_CTRL|");
 			c.id &= ~V4L2_CTRL_FLAG_NEXT_CTRL;
 		}