gve: merge packet buffer size fields

gve: merge packet buffer size fields

The data_buffer_size_dqo field in gve_priv and the packet_buffer_size
field in gve_rx_ring theoretically have the same meaning, but they are
defined in two different places and used in two separate contexts. There
is no good reason for this, so this change merges those fields into the
packet_buffer_size field in the RX ring.

This change also introduces a packet_buffer_size field to struct
gve_rx_queue_config to account for cases where queues are not allocated,
such as when the interface is down.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20250321002910.1343422-5-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>