mm/dmapool.c: fix null dev in dma_pool_create()

mm/dmapool.c: fix null dev in dma_pool_create()

A few drivers invoke dma_pool_create() with a null dev.  Note that dev is
dereferenced in dev_to_node(dev), causing a null pointer dereference.

A long term solution is to disallow null dev.  Once the drivers are fixed,
we can simplify the core code here.  For now we add WARN_ON(!dev) to
notify the driver maintainers and avoid the null pointer dereference.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>