macvtap: fix the uninitialized var using in macvtap_alloc_skb()

Commit d1b08284 use new frag API but would leave f to be used
uninitialized, this patch fix it.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>