powerpc/pseries: Correct VIO bus accounting problem in CMO env.

In the VIO bus code the wrappers for dma alloc_coherent and free_coherent
calls are rounding to IOMMU_PAGE_SIZE.  Taking a look at the underlying
calls, the actual mapping is promoted to PAGE_SIZE.  Changing the
rounding in these two functions fixes under-reporting the entitlement
used by the system.  Without this change, the system could run out of
entitlement before it believes it has and incur mapping failures at the
firmware level.

Also in the VIO bus code, the wrapper for dma map_sg is not exiting in
an error path where it should.  Rather than fall through to code for the
success case, this patch adds the return that is needed in the error path.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>