[SCSI] ibmvscsi: Correct DMA mapping leak

The ibmvscsi client driver is not unmapping the SCSI command after
encountering a DMA mapping error while trying to map an indirect
scattergather list for the event pool.  This leads to a leak of DMA
entitlement that could result in the device failing future DMA operations
in a CMO environment.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>