gve: update GQ RX to use buf_size

gve: update GQ RX to use buf_size

Commit ebdfae0d377b ("gve: adopt page pool for DQ RDA mode") introduced
a buf_size field to the gve_rx_slot_page_info struct, which can be used
in the datapath to take the place of the packet_buffer_size field, as it
will already be hot in the cache due to its extensive use. Using the
buf_size field in the datapath frees up the packet_buffer_size field in
the GQ-specific RX cacheline to be generalized for GQ and DQ (in the
next patch), as there is currently no common packet buffer size field
between the two queue formats.

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/20250321002910.1343422-4-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>