vtnet: allow IFF_ALLMULTI to be set without VIRTIO_NET_F_CTRL_RX

If the host doesn't announce VIRTIO_NET_F_CTRL_RX we cannot disable all
multicast traffic. Previously we'd refuse to set the IFF_ALLMULTI flag,
which is the exact opposite of what is actually happening.

This broke things such as igmpproxy.

See also: https://redmine.pfsense.org/issues/14301
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41356