gve: DQO: avoid unused variable warnings

The use of dma_unmap_addr()/dma_unmap_len() in the driver causes
multiple warnings when these macros are defined as empty, e.g.
in an ARCH=i386 allmodconfig build:

drivers/net/ethernet/google/gve/gve_tx_dqo.c: In function 'gve_tx_add_skb_no_copy_dqo':
drivers/net/ethernet/google/gve/gve_tx_dqo.c:494:40: error: unused variable 'buf' [-Werror=unused-variable]
  494 |                 struct gve_tx_dma_buf *buf =

This is not how the NEED_DMA_MAP_STATE macros are meant to work,
as they rely on never using local variables or a temporary structure
like gve_tx_dma_buf.

Remote the gve_tx_dma_buf definition and open-code the contents
in all places to avoid the warning. This causes some rather long
lines but otherwise ends up making the driver slightly smaller.

Fixes: a57e5de4

 ("gve: DQO: Add TX path")
Link: https://lore.kernel.org/netdev/20210723231957.1113800-1-bcf@google.com/
Link: https://lore.kernel.org/netdev/20210721151100.2042139-1-arnd@kernel.org/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>