ibmvscsi: driver enablement for CMO

Enable the driver to function in a Cooperative Memory Overcommitment (CMO)
environment.

The following changes are made to enable the driver for CMO:
 * DMA mapping errors will not result in error messages if entitlement has
   been exceeded and resources were not available.
 * The driver has a get_desired_dma function defined to function
   in a CMO environment. It will indicate how much IO memory it would like
   to function.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked by: Brian King <brking@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>