ioat: ioat_alloc_ring() failure handling.

If dma_alloc_coherent() returns NULL in ioat_alloc_ring(), ring
allocation must not proceed.

Until now, if the first call to dma_alloc_coherent() in
ioat_alloc_ring() returned NULL, the processing could proceed, failing
with NULL-pointer dereferencing further down the line.

Signed-off-by: Alexander Barabash <alexander.barabash@dell.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/75e9c0e84c3345d693c606c64f8b9ab5@x13pwhopdag1307.AMER.DELL.COM
Signed-off-by: Vinod Koul <vkoul@kernel.org>