net: page pool: allow to pass zero flags to page_pool_init()

page pool API can be useful for non-DMA cases like
xen-netfront driver so let's allow to pass zero flags to
page pool flags.

v2: check DMA direction only if PP_FLAG_DMA_MAP is set

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>