net: make default TX queue length a defined constant

The default TX queue length of Ethernet devices have been a magic
constant of 1000, ever since the initial git import.

Looking back in historical trees[1][2] the value used to be 100,
with the same comment "Ethernet wants good queues". The commit[3]
that changed this from 100 to 1000 didn't describe why, but from
conversations with Robert Olsson it seems that it was changed
when Ethernet devices went from 100Mbit/s to 1Gbit/s, because the
link speed increased x10 the queue size were also adjusted.  This
value later caused much heartache for the bufferbloat community.

This patch merely moves the value into a defined constant.

[1] https://git.kernel.org/cgit/linux/kernel/git/davem/netdev-vger-cvs.git/
[2] https://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/
[3] https://git.kernel.org/tglx/history/c/98921832c232



Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>