netlink-socket: Convert from error number to string correctly.

As mentioned in the comment above the function ovs_strerror(), it
should not be used to convert WINAPI error numbers to string.
Use ovs_lasterror_to_string() instead.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>