s390/qeth: implement smarter resizing of the RX buffer pool

The RX buffer pool is allocated in qeth_alloc_qdio_queues().
A subsequent pool resizing is then handled in a very simple way:
first free the current pool, then allocate a new pool of the requested
size.

There's two ways where this can go wrong:
1. if the resize action happens _before_ the initial pool was allocated,
   then a subsequent initialization will call qeth_alloc_qdio_queues()
   and fill the pool with a second(!) set of pages. We consume twice the
   planned amount of memory.
   This is easy to fix - just skip the resizing if the queues haven't
   been allocated yet.
2. if the initial pool was created by qeth_alloc_qdio_queues() but a
   subsequent resizing fails, then the device has no(!) RX buffer pool.
   The next initialization will _not_ call qeth_alloc_qdio_queues(), and
   attempting to back the RX buffers with pages in
   qeth_init_qdio_queues() will fail.
   Not very difficult to fix either - instead of re-allocating the whole
   pool, just allocate/free as many entries to match the desired size.

Fixes: 4a71df50 ("qeth: new qeth device driver")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>