ethdev: fix invalid length write on detach

The name of a device is copied in a provided buffer within
rte_eth_dev_detach(). The current sizeof is done on a pointer instead of
the intended array usually pointed to.

The name field of an rte_device is not assured however to point an
rte_devargs name field. The almost correct length to base this copy over
is thus RTE_DEV_NAME_MAX_LEN.

Almost correct, because unfortunately this function does not allow the
user to pass down a size parameter for the buffer it is meant to write.
This API should be fixed, it is broken by design.

Fixes: a1e7c17555e8 ("ethdev: use device name from device structure")
Cc: stable@dpdk.org
Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>