macvtap: Increase limit of macvtap queues

Macvtap should be compatible with tuntap for
maximum number of queues.

commit 'baf71c5c (tuntap:
Increase the number of queues in tun.)' removes
the limitations and increases number of queues in tuntap.
Now, Its safe to increase number of queues in Macvtap as well.

This patch also modifies 'macvtap_del_queues' function
to avoid extra memory allocation in stack.

Changes from v1->v2 :
Michael S. Tsirkin, Jason Wang  :
                  Better way to use linked list to
avoid use of extra memory in stack.
Sergei Shtylyov : Specify dependent commit's summary.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>