Revert "macvtap: remove useless codes in macvtap_aio_read() and macvtap_recvmsg()"

This reverts commit 41e4af69.

MSG_TRUNC handling was broken and is going to be fixed in the
'net' tree, so revert this.

Signed-off-by: David S. Miller <davem@davemloft.net>