netlink-socket: Fix nl_sock_recv__() on Windows.

In nl_sock_recv__() on Windows, we realloc a new ofpbuf to copy received
data if the caller specified buffer is small. While we do so, we need
reset some of the other stack variables to point to the new ofpbuf.

Other fixes are around using 'error' rather than 'errno'.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>