ethdev: fix data alignment

ethdev: fix data alignment

The struct rte_eth_dev_data is used in ethdev fastpath routines
and it not aligned to cache line size. This patch fixes the ethdev
data alignment.

The alignment was broken from the "first public release" changeset
where ethdev data address was aligned only to the first port.
Remaining ports alignment was defined by the size of the struct
(rte_eth_dev_data). This scheme is not guaranteed to be cache line
aligned all the time.

"ethdev: add port ownership" change set introduced a
rte_eth_dev_shared_data container for port ownership change,
This resulted in rte_eth_dev->data memory for the first port also
as cache unaligned.

Added a compiler alignment attribute to make sure
rte_eth_dev->data always cache aligned so that CPU/compiler
1) Avoid sharing the element with another cache line
2) Can load/store the elements in struct rte_eth_dev_data as
naturally aligned.

Some platform like thunderX could see performance regression of 1%
at "ethdev: add port ownership" change set with
1 port/1 queue l3fwd application and this patch fixes that regression.

example command:
sudo ./examples/l3fwd/build/l3fwd -c 0xff00 -- -p 0x1 --config="(0,0,9)"

Fixes: af75078fece3 ("first public release")
Fixes: 5b7ba31148a8 ("ethdev: add port ownership")
Cc: stable@dpdk.org
Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>