ibmvnic: Reorganize device close

Introduce a function to halt network operations and clean up any
unused or outstanding socket buffers. Then, during device close,
disable backing adapter before halting all queues and performing
cleanup. This ensures all backing device operations will be
stopped before the driver cleans up shared resources.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>