ipx: Remove spurious NULL checking in ipx_ioctl().

We already unconditionally dereference 'sk' via lock_sock(sk) earlier
in this function, and our caller (sock_do_ioctl()) makes takes similar
liberties.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>