xen-netback: avoid allocating variable size array on stack

Tune xen_netbk_count_requests to not touch working array beyond limit, so that
we can make working array size constant.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>