gve: support unreadable netmem

gve: support unreadable netmem

Declare PP_FLAG_ALLOW_UNREADABLE_NETMEM to turn on unreadable netmem
support in GVE.

We also drop any net_iov packets where header split is not enabled.
We're unable to process packets where the header landed in unreadable
netmem.

Use page_pool_dma_sync_netmem_for_cpu in lieu of
dma_sync_single_range_for_cpu to correctly handle unreadable netmem
that should not be dma-sync'd.

Disable rx_copybreak optimization if payload is unreadable netmem as
that needs access to the payload.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20250818210507.3781705-1-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>