net: skbuff: move alloc_cpu into a potential hole

alloc_cpu is currently between 4 byte fields, so it's almost
guaranteed to create a 2B hole. It has a knock on effect of
creating a 4B hole after @end (and @end and @tail being in
different cachelines).

None of this matters hugely, but for kernel configs which
don't enable all the features there may well be a 2B hole
after the bitfield. Move alloc_cpu there.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>