gve: fix -ENOMEM null check on a page allocation

Currently the check to see if a page is allocated is incorrect
and is checking if the pointer page is null, not *page as
intended.  Fix this.

Addresses-Coverity: ("Dereference before null check")
Fixes: f5cedc84 ("gve: Add transmit and receive support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>