nfp: handle page allocation failures

page_address() does not handle NULL argument gracefully,
make sure we NULL-check the page pointer before passing it
to page_address().

Fixes: ecd63a02

 ("nfp: add XDP support in the driver")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>