Unabuse struct tcb.auxstr

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.