af_packet: fix sparse warning

af_packet produces lots of these:
	net/packet/af_packet.c:384:39: warning: incorrect type in return expression (different modifiers)
	net/packet/af_packet.c:384:39:    expected struct page [pure] *
	net/packet/af_packet.c:384:39:    got struct page *

this seems to be because sparse does not realize that _pure
refers to function, not the returned pointer.

Tweak code slightly to avoid the warning.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>