ospf6d: read ospf6 socket until failure

ospf6d: read ospf6 socket until failure

To ensure we read all the datagrams availabe from a socket when the
read task is scheduled, make the read helper return and error or
continue enum and loop unitl an error is received.
This requires the read from the socket to be non blocking

Signed-off-by: Pat Ruddy <pat@voltanet.io>