Don't log "Unexpected netlink message" on NDUSEROPT

When opening the netlink socket, NetlinkManager specifies it's
interested in ND_USEROPT messages, but we since we don't have
code to parse them yet, we end up logging an error message for
packets that contain them. Get rid of the logspam by not asking
the kernel to receive them.

Bug: 10718651
Change-Id: Ib1b7748448a983cfa7bb7725e48e238d85152ea2