tun: remove useless codes in tun_chr_aio_read() and tun_recvmsg()

By checking related codes, it is impossible that ret > len or total_len,
so we should remove some useless codes in both above functions.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>