net: repack struct netdev_queue

net: repack struct netdev_queue

Adding the NAPI pointer to struct netdev_queue made it grow into another
cacheline, even though there was 44 bytes of padding available.

The struct was historically grouped as follows:

    /* read-mostly stuff (align) */
    /* ... random control path fields ... */
    /* write-mostly stuff (align) */
    /* ... 40 byte hole ... */
    /* struct dql (align) */

It seems that people want to add control path fields after
the read only fields. struct dql looks pretty innocent
but it forces its own alignment and nothing indicates that
there is a lot of empty space above it.

Move dql above the xmit_lock. This shifts the empty space
to the end of the struct rather than in the middle of it.
Move two example fields there to set an example.
Hopefully people will now add new fields at the end of
the struct. A lot of the read-only stuff is also control
path-only, but if we move it all we'll have another hole
in the middle.

Before:
	/* size: 384, cachelines: 6, members: 16 */
	/* sum members: 284, holes: 3, sum holes: 100 */

After:
        /* size: 320, cachelines: 5, members: 16 */
        /* sum members: 284, holes: 1, sum holes: 8 */

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240820205119.1321322-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>