[SCSI] ibmvfc: Fix DMA mapping leak on memory allocation failure

There is currently a DMA mapping leak that can occur in the ibmvfc
driver if we fail to allocate a scatterlist. Fix this by unmapping
the scatterlist in the failure path. Additionally, only log an error
for a scatterlist allocation failure if the log level is greater
than the default, since this can occur when running Active Memory
Sharing and this is not considered an error.

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