gve: Allow ethtool to configure rx_buf_len

gve: Allow ethtool to configure rx_buf_len

Add support for getting and setting the RX buffer length via the
ethtool ring parameters (`ethtool -g`/`-G`). The driver restricts the
allowed buffer length to 2048 (SZ_2K) by default and allows 4096 (SZ_4K)
based on device options.

As XDP is only supported when the `rx_buf_len` is 2048, the driver now
enforces this in two places:
1.  In `gve_xdp_set`, rejecting XDP programs if the current buffer
    length is not 2048.
2.  In `gve_set_rx_buf_len_config`, rejecting buffer length changes if XDP
    is loaded and the new length is not 2048.

Signed-off-by: Ankit Garg <nktgrg@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251106192746.243525-4-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>